I paid £4,800 a year ago, the vendor had wanted £6,000. Mine is a 1972 Gt Junior, same bodyshell as the 1750 GTV but with single headlights (still has black grill with stainless strip), interior is a cheaper version of the 1750 GTV but doesn't have carpets (mold gathers), a central wooden console (a bit cheap looking in my mind) or the seats with a backrest for the passenger only, it also has a plastic steering wheel. The 1300 engine has been replaced with a 2000GTV engine, as well as the back axle/diff which means I also have a limited slip diff. My brakes are also from the GTV twin servos. The bumpers are also stainless steel, it has smaller rear lights and no rear fog or reverse lights. 1972 is the last year for tax exempt, my cousin had a 1973 MGBGT on a M plate which failed to get exemption.
I've had some rust removed in the last year, but have thought it that expensive, and most replacement panels are readily available.
I would think the mechanics (very little and simple electrics) if looked after are nearly as reliable as modern stuff, if its not going to be your main car you won't drive it when you shouldn't, i.e. low on oil, bad weather etc.