Sounds like you have all the usual symptoms...
Mine has a Lockheed replacement servo and has a long-ish pedal. It's been much better since tesing the brakes to the limit on the MoT test last week though! They might need bleeding and/or new fluid if it hasn't had much use (a lot of them stand idle for months at a time which does them no good at all...
). Or it could be extra slack wheel bearings (front) or disc runout knocking the pads back.
Is it a 5-speed? If so, they usually show signs of wear if they haven't been rebuilt or very carefully looked after. Commonly it is 2nd gear that is difficult to select, or 1st & reverse if there's a clutch 'problem' (drag due to pressure plate hub contact, or poor disengagement). I've not come across a problem with 4th before, but 5th can be tricky to select as the lever is not sprung loaded like a modern car - it takes a bit of getting used to...
Vibration - does it have alloy wheels? The originals are difficult to balance ('oh no, not THOSE again...' says the man at the garage!). Being a relatively lightweight car it is very sensitive to the imbalance. My +2 had quite a lot of vibration at the back at 60mph. After careful wheel balancing with new tyres, I changed the propshaft (a common cause as you suggest) for one with less wear in the sliding spline, and fitted new universal joints. I also changed the gearbox, diff mountings, dampers, & driveshafts. Now I can reach 70mph+ before a 'mild' vibration starts!
Vibration van also be caused by the diff contacting the bottom of the chassis, usually during engine braking, perhaps due to worn diff top and/or bottom mountings.
It's worth checking the rubber 'donut' drive couplings as they can fall apart with violent use or just over time. I watch mine when they jack up the chassis on the ramps at the MoT garage. The suspension hangs at full droop and there's usually a nasty splitting sound when the wheels are rotated as the rubber begins to come away from the metal plates. It looks bad, but they can last a while if it isn't used for track days, etc. If you've any doubts though, replace them as they can make an expensive mess of the rear of the chassis/body...
Unless you buy a 'perfect' one, or you take everything apart & rebuild it, you will probably have many 'happy' years of discovering the short-cuts & poor maintenance of previous owners... I have, but the more problems you sort out, the better they get so HAVE FUN
Matthew