Welcome to the best Nissan forum around! Definitely post some pictures of the new (to you) 240! Most engine mods don't cross over between the single cam KA24s and the dual cam KA24s (91+). The test pipe and catback exhaust are interchangeable, but from the intake tube to the intake manifold, head, and exhaust manifold are all different. The block is similar, but there are some differences. What state do you live in? Do you have emissions laws? How strict? The reason why I ask, is because this will dictate what sort of options you have in terms of engine mods or swaps you can do. Also, before starting out doing modifications, what kind of budget do you have and what are your plans for the car?
As for the issues; 240s are notorious for having sensor/electrical/vacuum leak issues which cause them to run less than ideal. Using the factory service manual you can run through various diagnostics to find out your issues. Here's the link to the FSM:
http://www.nicoclub.com/archives/nissan ... nuals.html
The transmissions are also known to be fairly worn with a lot of miles. You can try your luck with a used transmission or rebuild the one you have. Used transmission's go for pretty cheap ($200-$400 range typically). The clutch squeak is most likely do to the pedal needing a little lube. Engine probably shakes due to low RPMs correct? Might want to check out the motor mounts.
Anyways, welcome aboard!Hopefully more can chime in.