1300 crossflow is a pretty decent engine. not a rocket by any means, but pretty decent and revs well and don't foget how light the car is. TBH not worth spending a lot on mods. You will get an instant boost by swapping for a 1600 crossflow and spend money on that. You can swap for a 2.0 Pinto, but you need to swap the exhaust routing to the other side. Zetec conversions are reasonably popular but they start to get expensive. A 5-speed gearbox swap is also a common mod.
$1800 is a bargain as long as it's reasonably decent. if it's a matching numbers car I'd be keeping the 1300. Good models can easily top $30,000
anything can be done to the body
I've seen utes, but tbh if you really want a 2 door then it would be massively easier to buy one instead of the 4-door. 2-doors are in more demand for rallying and racing as the body structure is stiffer, but for road use you wont notice.