You did find a great vintage bicycle. It doesn't have the original components on it but that can be taken care of later if you want. The rust problem is, well a problem. You need to address as soon as your financially able to do so, and to take care of it the best way would be to strip the paint and have the rust removed then repaint it the same color of course. Aftermarket decals are available for that bike and cost around $35 for the complete kit. There are bicycle paint places around you could send it to but they'll start at around $250, sometime though a local bike shop may know of a good local painter that may do it for around $150, or if you know how to do that sort of thing yourself then save even more money.

You may be thinking why go through a complete repaint? first because the bike is more then worth it, that alone would merit a new paint job; and second you need to get that rust completely removed and you can't do that with a treatment then touch up paint, it will look bad afterwards; and the cable guide might, note I said might, need to be rebrazed which will burn the paint. Stripping it down will also allow the painter to look at other brazeons to make sure their ok.

Do not have it powdercoated because powdercoat goes on thicker then regular paint and will reduce the look of the lugs because the thickness of the paint will reduce the detail of the cut of the lugs so that they look very minor instead of sharp.

By the way, I own a 84 660 that I bought new in California back in 84 and it's been a great bike for over 150,000 miles and has no rust anywhere not even on the cable guides. I knew several people over the years that had various 80's era models of Trek bikes and none of them had rust on the guides, so I'm not so sure about the commonality of rust being found on the guides of those bikes.

But I would worry about the rust and repaint as soon as possible, then later, if you want to restore it more fully, see a Trek catalog on the Vintage Trek site and get a list of the original parts and try to find them on E-bay over time. Like I said, this bike is worth the repaint. I couldn't find any 760's on E-bay today but a 560, two steps down from yours is at $329 with 4 and half days left; I did see one of yours in good shape with original components go for $1100 about a year ago.