The Trek looks like a 58cm (23") frame. It may very well fit you properly, but you need to do the "stand over" test. Make sure you can lift the wheels about an inch off the ground when straddling the top bar. Make sure you feel comfortable on it. The back wheel is going to be out of alignment, due to that broken spoke. Test riding might not be that easy...have a spare 27" rear wheel handy? The 21" frame might just be too small. Worth checking out both, if you can get a chance to. Make sure the Trek is in original condition as much as possible. Plenty of info and specs on Treks here .... including copies of original sales brochures ....

http://vintage-trek.com/

The Trek seat tube decal looks older than '84/85, so he may have indicated the wrong model number. The 420 was towards the starting point of Treks line-up, although it would have been higher up in the hierarchy for other manufacturers. You might have something there. Old Treks, in nice condition, aren't too shabby a ride. Could be quite impressive, depending on condition.