first off you can't get the bluebird motor, its fwd, unless you have a shit load of money to spend. you can get either one of the SR's or the CA, they both fit with no problems.
Your best bet on those motors for the car you are looking at will be the S13 SR20DET. I see a lot of the CA18DET in the salvage yards in Germany, plus they are an older motor. There are also a lot of aftermarket parts for the SR20's. So parts wouldn't really be a problem.
if you are talking about getting a model 90-91 240sx then don't get the blue bird. If you are talking about a 90-91 200sx you are in the wrong forum.
But basically ca18det is a 1.8 little small for a 240sx. If you have the money go with the sr20det there is no replacement for displacement. Supposedly it is one of the easier swaps but I have never done it I have just heard it is. But if you got more money go with an rb26 or 25det it is a straight six that has the potential to become a supra killer.