Hmmmm, one I can answer.
1: Maybe. 20 series came out with two different diffs in them. If it is original, the 25 will have a Jap style front loading diff, whereas the 20 may have either a Jap, or a Borg Warner diff. A Borg has a plate on the back of the housing. Quick tip, if you look under the back of the car, and the housings look the same, they'll be interchangable.
2: Incorrect. KE30 diffs are not stronger, as they are the same as KE20 diffs. You can put the brakes on you car (Irokin did it to his) but you also change the stud pattern and your current wheels wont fit.
3: Yes it will fit, however you will need very small offset wheels if you don't shorten the diff. Celica stud pattern is also different to KE20, so once again, wheels won't fit. Lipping guards is a must. Axles aren't a whole lot stronger, but the diffs are good.
4: depends on how handy you are. Will it fit? Yes. Will it all bolt in? No.