It's finally tax return season (aka bike buying season) and I'm setting aside $600-700 to put into a new fixed gear. I've had a generic frame I built up with Weinmann's and other mid-low level components that I was moderately happy with, so I want something nicer. I don't want or need to exceed 700 bucks.

To get to the point: I'm looking at the Mark V Pro's because they use Reynolds 631, but how much better is that than the IRO heat treated CroMo? I don't have any experience with IRO bikes, or any steel for that matter. IRO's build a bike website is out of the 58s at a moment - they have the Mark V in a 56. I'm 6'1" and rode a 56 with drop bars before and was quite comfortable (my TCR is a M/L 57 - I have a short torso and prefer to ride with more seatpost). It was also easy to get my weight over the bars. How do the Mark V's feel as far as reach?

What other bikes in this pricepoint should I be looking for? I'll mostly be using the bike for training in the rain, maybe a couple flat centurys and having a non-carbon bike to get around town on.

IRO makes good stuff. This is a great bike to buy. You will get solid performance out of a reasonably priced bike.

As far as 631 vs. basic CrMo goes, there are some differences. 631 is a bit stronger meaning that it can be drawn thinner and is thus lighter. I have a bike made out of 631 (Jamis Sputnik), and I absolutely love the way it feels. Something about it seems a bit snappier, too. When i mash up hills, it seems like the bike kind of bounces off of each pedal stroke. I dono, maybe its all a placebo effect and I'm a product of clever marketing, but I can notice a difference between it and my standard CrMo 4130 ride.

Speaking of Sputnik, you might want to check one out. I like mine a lot. Salsa Casseroll is also nice, IMO. Steamrollers are always good (though I think they are overpriced). These are all more relaxed geo bikes. If you want something tracky, there's another list...

I am an owner of an IRO Mark V and I'm very happy with it. (although I still think it was stupid to pay so much for the "custom" options.)
I've been riding it for about half a year now, and nothing bad has happened to the frame (even after 3 crashes, one more serious at high-speed). Everything is still intact. I basically used it for commuting and occasionally skids. However, I can't speak on its performance if your're FG freestyling with it.

But if you dig in the forums and read some more, you'll find that there is no weight advantage between an IRO Mark V vs the Pro The ride quality may be improved however, like PedallingATX mentioned.
I would pay the extra $50 to get the Pro just because Reynolds has a very reliable reputation.

You should also consider the Angus, geometry should be the same, but imo, looks better because it's 1" threaded so you can put on a quill stem, and has no cable/ water bottle bosses (pro version available too).

I've got one of the FGSS group buy IRO frames, that are pretty similar to the Angus but made with Deda tubing. Mine has a threaded fork and is drilled for a rear brake (why not?). I think it's a great frame, and I wouldn't hesitate to recommend the company. I just got the frame and fork, then put my own components on it, which is what I would do if I were you since I don't think there's anything special about the components they source and supply.

IRO frame = ~275
Open Pro+Formulas+butted spokes=250
All-City crank+1/8" chain+1/8"cog+lockring=140
Scavenge an alloy seatpost, solid-feeling stem, handlebars, brake caliper and lever from a co-op or the used bin at a LBS (mine gives away that kind of stuff)
I don't know what your preferences are for pedals and seat- I'd go clipless and Brooks, but you'll definitely be over budget with those.

That'd be a nice bike. You want to put the money where it'll make the biggest difference- rims, decent chainring, good solid frame. If you wanted to cut costs you could probably lose the nice butted spokes, maybe cheap out on the cranks themselves and just buy a good track chainring- those things are worth the money up front though, I think.

EDIT: https://www.bicyclewheelwarehouse.com...&productId=194
You can get the wheels for $205, with your cog and lockring. That frees up a bit of money for your saddle and pedals. Do it!