Your estimated costs... prepare to pay twice that if not more.
You cant just swap engines like you swap your undies, its illegal. If the engine didnt come in the car in the first place, it has to have an engineers cert to certify it meets emissions standards. The only 25 you can drop in is the non-VCT rb25de out of 32 gts25s which are harder to come across and ultimately a huge waste of time.
Coolant leak? Sounds like a broken radiator pipe, easy fix. Broken radiator? $300-$500+
If the car is already a colour and you respray it the same it can reduce the cost, but in your case its not. Front and rear bumpers , full sand down , and a DECENT full body respray = $3000+ atleast. Fair chance the things got rust in it too so theres more work to be done.
IMO, sell the car. You probs wont get 2500 for a rb20de auto skyline with coolant leaks, f**ked bumpers and shit paint tho, ide pay $1000 max.
Unless youre experienced, dont try and attempt any of what youve set out to do, its not worth it, will make you hate the car. Not trying to bring your hopes down or anything but things like paint jobs and such need to be done right or they look shit and last no time at all.
my 2c