how strong do the parts need to be - ie: what kind of stress does the most stressed part take and in what directions.

Fdm printers can make very strong, durable and practical parts (99% of all my prints are practical).
But for some things a resin printer using a 'tough' resin might be more suitable.

Oh and start thinking and working in millimetres. Pretty much all the 3d printing specific software only works in mm.

If you look on thingiverse there are fully 'working' 3d printed car engines. So the fitting things together is not going to be the issue.

Choosing the right material to make the parts from is probably more important.

Also how you design the parts makes a HUGE difference.

Parts designed specifically to be 3d printed will be stonger and mush easier to print than parts you just designed without taking 3d printing requirements into account.

Probably the best suited to your purposes at the moment would be a flashforge replicator pro 2 - ungortunately It's pretty new and i can't find a uk distributor.

How 'handy' are you with mechanical things ? ie: would you be happy with a build it yourself kit ? as opposed to a ready built machine ?