http://www.vintage-trek.com/images/t...listApr_82.pdf
According to the 1982 price list, a 710 frame would have been $349 for the frame and assuming a good component group $409 on top of that.
Keeping that in mind- Using an inflation calclulator, that $750 you spent in 1982 would roughly equate into an $1800 bike today.
IMO, I love old Treks. If I really wanted one, I'd spend what I thought would be necessary to buy one. $400 would be absolutely as much as I would pay for an
immaculate condition 1982 Trek 710 with a really nice component group. I don't think I'd even really pay that much.
I would not pay in excess of $750 and definitely not $1800 nor $2100 for one.
You've got a desirable bike- if you want to build it up to ride, or to sell- it's going to be a really cool bike and a great riding bike.
If'n it were me, I'd build it up with period Suntour Cyclone stuff with Superbe brakes and brake levers.
Like this style:
This is just *my* personal opinion... if you're selling it, go all vintage, or go all modern. IMO piecemealing together a group looks like ... parts put together to make it run-able.
If you're going to ride it, use whatever makes you happy and don't give a rat's ass what some dork on the internet thinks.
Best!