Firstly, welcome to the Forum.
Please fill in at least a basic Location in your Forum User Profile, this'll appear automatically beside all your posts, helps responders if advice needs to be location-specific. Imho, this is one of those occasions ...
the engine sounded great but was rather oil smokey on start up,
Would that be normal for a brand new rebuild or could something else, such as valve guides be an issue? If so, is this a*big job?
Errrm ... if you don't know, depending on your location, this is probably not the bike for you.
There can be all sorts of reasons a Britbike smokes, none of 'em good. If, say, you're in GB, and you aren't limited to a particular version and/or year, T140 is not exactly a rare bike, you don't need to buy one with obvious problems. The only two exceptions I can think of are:-
. if the bike's being offered at such a low price you've enough money left - and you don't mind learning - to do all the alleged top end work over again;
. your location is so remote (Australian Outback?), T140's - smoking or not - aren't very thick on the ground ...?
Ime, to avoid disappointment, it's wise to treat any seller of a forty-plus-year-old vehicle you don't know as a complete idiot; if you find out subsequently he/she had some mechanical ability, it's a pleasant bonus.