Series 3 my swb series 3

am refurbishing a series 3 and was thinking about replacing axles engine, etc however i am now thinking that a better idea would be to buy a simple first car and restore it as a classic any opinions would be great.

You wont get anything much simpler than a series Landie unless you go pre war.
Reading your other post re Rangie as a donor car, forget using the axles. Track is to wide and coil sprung, it is possible but a serious amount of work/welding spring mountings etc. I would try as a first rebuild to just do a tidy near standard refurbish with maybe a 200 TDI up front, plenty of threads on this. Try looking at Glencoyne's web page for info on 200 engine fitting.

would adding the 200tdi ruin its authenticity

You won't get much simpler than a landy, particularly a S3 from the 70's. Equivalent cars of this era are now all largely collectable and whilst mechanically as simple as a landy they command higher prices unless you buy a clapped out heap. Good starter cars would be escorts or something from the BL stable (but not a Allegro - the world's worst car!). Suspension, electrics and engines are all quite similar. I'd try to get my hands on a slightly tired genuine Mini, dead easy to work on but watch for rotten sills.