I bought a very used mountain bike - Trek 3700 - to ride the numerous gravel roads, and occasional trails in my area. I got it rideable but to really finish it will take about $100. That would be new cables, bars and stem, chain, and pedals.
Here’s my dilemma, with $50 in and $100 total I could have a competent bike, albeit a 3X7 - OR - for $225 a new bike from Bikesdirect that comes with a 1X7 drivetrain?
Opinions please, although I think I made up my mind, but would like other’s point of view.

I'd finish the old Trek without question. There's something cool about making it your own...

Do these bikes have freewheel hub or cassette hub? I am guessing the Bikesdirect bike has freewheel hub and the Trek 3700 cassette hub. Cassette hub is much better than freewheel hub.
Also, go to this website :
Bicycle Gear Calculator
and enter in the bikes' chain rings and cogs information, and you can see exactly what gear ratios each bike offer. I am guessing the 3X7 will give you a bigger gearing range than the 1X7.

$225 "New Bikes" suck... finish building the trek.

As to your first point - the Trek is a 2004 model so I doubt if it is a Cassette. And the Gravity's parts list specs that it is both a freewheel and a cassette (?).
And as to the "21 speeds" riding flat gravel roads in the nearby LBL seldom requires more than just two gears.

I will explore the link just to be better informed about the bikes.

I stand (sit) corrected - the Trek is indeed a cassette.

For the money I'd opt for the TREK. $100 is a good investment to make on a previously owned bicycle if you're buying stuff to make it fully functional once again as well as safe to ride.

Likely as not, spending = $$ on a new bike won't bring you an = ride quality. Besides that, resurrecting something someone else's tossed to the curb brings some gratification in the doing as well as whatever eventual pleasure you get out of using it.

And you'll learn some things along the way, maybe even something new to you.

I think it depends how serious you are about gravel riding. If you are going to just ride gravel every now and then I would stick with a Franken 3700 and be happy with it. But if gavel riding was going to be my main rides then I would consider selling the 3700 and taking the money from it plus $100 USD and go for a 400 USD "Gravel Bike"... Yep...

If you haven't ridden your current bike very much, then ride it. Ride it a lot. Till you know what it does or doesn't do for you. Then get another bike that solves some of those things that the old bike can't.

Personally, I'm not certain what the new bike will solve that the old bike can't do. But I may not have compared the same models you are talking about.

Realize also that a mountain bike is generally made for where only mountain goats can go. Off of the paved or gravel road. That's why they got the name mountain bike. Road bikes work every bit as well on mountain roads. Paved or gravel.

Is this the Bikesdirect bike you were eyeballing:
https://www.bikesdirect.com/products...tain-bikes.htm

This bike uses a freewheel hub. Do you know how I know? Because it lists the freewheel as SHIMANO MF-TZ500 14-28T-7speed, and that is a freewheel, not a cassette. In Shimano nomenclature, MF-**** is a freewheel (or 'Multiple Freewheel'); and CS-**** would be a cassette.
BTW 14-28 is a pretty narrow range. So for sure, the 3X7 trek bike will have a much wider gearing range than this 1X7 bike. Just fix up the Trek and forget about the cheapie 1X7 bike.

It is indeed. And thank for lesson on Shimano freewheels/cassettes.

And to all the others who recommended I keep the Trek, you have convinced me to do so.
I realized that the new bike would not be an upgrade.

Thank you all.

This.
You already acquired the brand-name bike and got halfway through fixing it up yourself. Stay the course.

Exactly true yet riding varying surfaces to my mind requires a proper set of suitable tires.

I have a second set of wheels with strictly pavement tires mounted on 'em, otherwise the set that's on my #1 road bike most of the time are suitable for both pavement and packed gravel.

Better bikes have had cassette hubs since the late 1980s, although there were still plenty of high quality freewheel hubs too - Campagnolo and Mavic were slower to adopt freehubs than were Shimano and Suntour.

Two main points:

• Old rigid mountain bikes are very suitable as gravel bikes. I like "butterfly" handlebars for the various positions they provide while retaining the original controls.
• None of the cheap Bikesdirect bikes are bargains - they tend to be uniformly nasty "bicycle shaped objects" that are not suitable for actual cycling.

​​​Also don't make the mistake of thinking that narrow high pressure tyres are faster than wider softer tyres - a good quality supple wider tyre will improve ride quality without sapping power, while a hard narrow tyre will be tiring on any but the smoothest of surfaces.

a typical Trek 3700.. A Hardtail with front suspension and MTB tires, stock.
https://content.propertyroom.com/lis...1531357984.jpg
most of the older ones have v-brakes.

Excellent choice OP.

A bit of information I have learned: The Trek 3700 has a freewheel. Oddly numbered - TZ37 - with the TZ designation indicting a Freewheel.
Confusing as the Trek data sheet for the 3700 specs the Cassette as a TZ37 which googles to:

"The Shimano TZ37 is a freewheel. It's a more basic type of rear gear system, often found on entry-level bikes."

Not a limiting factor for what I intend to do with the bike.

Well now that you know what to look for, you can sell the Trek 3700 and look for a better MTB to buy instead of sinking money into that

Nope - I’m going to finish the Trek, and ride it until it limits me in what I want to do with it. Then, and only then, will I look to replace it.

I know you have decided to stay with the Trek. But I'll reinforce that probably any Trek that is rideable is going to be better in almost all aspects to any new bike costing $225. While I'm not familiar with that Trek, all in for $150 sounds great. And I think your approach of ride it until it no longer meets your needs is a great way to approach this.

Ah, my old 3900 came as a bare frameset with a rigid disc fork, but I just searched through catalogs and it originally had Suntour springs. It's from 2009, newer than I thought, with both disc and rim brake fittings. I built it into a sort of gravel cruiser thing, Schwalbe Fat Frank on cross country wheels, discs and 2x10, backswept riser bar which I need to replace, my wrists don't like it, I should try it with drops.

swap on a shorter stem to help the wrist thing? worth a try.. this helped me.. it reduced the stretch, and occasional locking of my elbows... the bent arms gave me more upper body suspension and lessened bump shock...... the drops will be WAY too far away, and will call for a stubby stem, ala downhill stuff...
i went from a 100mm to an 80 on my Sirrus... helped the numb palms thing quite a bit... as did Oury large dia. grips... i still need to narrow the bars a touch too... maybe soon... springtime is busy busy time.

heck, run a bingoogleduck search for "reducing wrist pain while bicycling".. see what pops up.