1,
if you have the hole complete airbag suspension kit,
(that bolts right up)you should be able to do it yourself,
(maybe with a book on it or the internet would be best!)
I have a friend w/a 4g4d with airbags,w/no mounting brackets,
he just took metal & welded it all together,(no problems)
I'm hoping to get airbags soon myself
2,
if your engine has 225,000?
you could rebuild it,which would be better,but more money,
pistons+rings,crank kit,oil pump,valves,timing belt,water pump,ext....
will cost you well around 600-800 $,
or like you said,just swap w/another motor,it would be your best bet!
if your not for speed,go for the cheapest you can find,
on the check engine light,lift up the passenger side carpet,under the dash,you'll see a gold plate,w/a clear circle in the middle,put your key in & turn it once,you'll see it beap once,it shouldnt beep no more after that
if it does,count how many times it beeps & post it back on here,
we'll let you know what it is,
(that could be your problem for not starting somtimes!)
3,
no
but I'm hoping someone here has done the swap!
good lucc