Welcome to the forum.
There isn't a simple answer. You will need to take into account how you will be using the car, what your budget is and your skills. Do you want a project, a good car (restored or original) or something in between (an abandoned project which is 90% done or suchlike)? You've already come to the right place and it is worth joining the club to take advantage of the benefits on offer. Try to attend a local meeting if you can then you can see what the cars are like.
You seem to be aware of the rust issues (all cars of the same era had the same problem). Electronic ignition is worth considering and has improved reliability no end on my 1850. There are some poor quality parts so it is worth paying that bit extra for decent quality. Overdrive makes for more relaxed top gear cruising so it is worth getting a car with overdrive if you can. It was the 1500HL, 1850/1850HL and Sprints that could be specified with overdrive from new although some owners have retrofitted an overdrive gearbox to the "lesser" cars. I helped out on such a conversion on a 1500SE about 10 years ago. Automatics are worth considering and the general consensus is that the 1850 is the best combination with the automatic transmission. The transmissions are quite reliable and the only problem is that the flexplate can break in the middle (I've had it happen to me twice in the 8 years I've owned my 1850 auto) which means the engine and transmission have to be separated to replace the flexplate.
Hope this helps.
West Yorkshire Area Organiser & forum moderator
Meetings take place on the first Wednesday of the month at 8.00pm at The Old Brickworks, Wakefield Road, Drighlington, Bradford, BD11 1EA
Dolomite 1850 auto (NYE 751L - Waiting for me to give it some love)
Citroën C4 Grand Picasso 2.0 HDi Exclusive (MA08 WCL - the workhorse)
BMW 318i (M265 PNC - Project Bimmer, the 2nd car)
Toyota Celica GT (J481 ONB - the current project car)
Former stable of SAY 414M (1974 Toledo), GRH 244D (1966 1300fwd), CDB 324L (1973 1500fwd), GGN 573J (1971 1500fwd), DCP 625S (1977 Dolomite 1300) & LCG 367N (1975 Dolomite Sprint) plus 5 Acclaims and that's just the Triumphs!
Check my blog at http://triumphtoledo.blogspot.com
"There is only one way to avoid criticsm: do nothing, say nothing and BE nothing."