Read through the links in the first post I made, they cover all the questions you just asked.

To give you the short answer...

1. Resetting your rear leaf springs is the best way to lower the rear of your car, and its quite a cheap and easy mod.
2. Coilovers for the front are slightly more involved and more expensive too. Put simply, its height adjustable suspension.
3. Tuning your a12 with some extractors/exhaust, a downdraft Weber and electronic ignition will wake up the stock engine, plus they're all reasonably cheap and easy. If I were you, I'd do those first while you source your new engine.
4. 13's are lighter which will give you better handling, although it can be hard to fit and brake upgrades. Work out what you want to do for coilovers/brakes before getting too set on wheel size. Personally, I'd try to get stuff to fit under 13's

Look through tyre catalogs. Seems to be more selection in 13 than 14 here in aus for performance tyres

For sure I'm getting a new carburetors. Weber conversion seems to be definite. I'm having my few friends getting it for me while I'm looking for new seat. TakInc my back seat completely out to put something else in.(its completely destroyed and I'm not looking to have people jumping in the back anyways.)