Classic and Vintage Bicycles: What's it Worth? Appraisals and Inquiries - Trek with all 600 components

I went ahead and bought an older Trek the other day, it was my size but it needs some work. For the most part it needs new cranks and wheels.
The frame has a 531 sticker, the wheels are Rigida rims with Shimano hubs with little oil clips on them. It's dark blue, and at 25", it's a perfect fit for me.
I gave $200 cash, it was listed at $350 in a local paper and there were three others there looking it over but I got there first. One other buyer offered me $300 on the spot for it before I left. The other guy stomped off mad about it being sold.

How much is too much to dump into this bike? I figured that since I got it cheap, I'd be safe to maybe put another $300 or so in it?
I looked around and most older Treks are listed for $400 or more, some way more. Even if I replace everything on this, plus a wheel set and labor, I should be able to come in under $500 or $550 or so total?
Also, How hard is it to get the shifters mounted up on the stem?
I sat on the bike the way it is but my beer gut keeps me from being able to reach the shifters way down low. If not on the stem maybe on the top tube?

Its hard to tell what model it was with Trek, you need to take the serial number over to Vintage-Trek.com and see figure out what that is. It may or may not have been all 600 equipped from the factory.
For it to be Reynolds 531 tubing I'd expect it to be a 616 or 716 and from what I can tell by that pic, its most likely a pre 81 model.
How about bar end shifters or convert it to a straight bar with thumb shifters? I did that with a couple bikes and it worked out well. There's nothing like a light upright riding bike, especially if it's got fenders and a comfy seat.
Finding rims for that may be an issue, most has Rigida rims and 700C Rigida rims are scarce these days.