well, the most inexpensive turbos are of course the stock ones. And, if you dont really plan on racing the car (we all said that at one point) I would just throw a stock one back in. The pros of stock are: easy to find parts for, inexpensive, as everybody is always ditching theirs in favor of bigger ones, quick spool up due to the smallish size, no question as to whether or not itll work out with your present combination, and it helps keep the originality, in case you want to sell the car.
Cons: Only flows soo much air (good for 12's) and starts to get really inefficient at around 20 lbs of boost already. The way I see it, the stocker has more pros than cons anyday. Plus, if you were to basically get any other turbo but stock, you may need to change some other things around. More hassle than anything. Just stick to the stock arrangement for now, and tune what ya got, till you get a bit more knowlegeable, then you can start throwin parts all over the garage later. Remember, if ya change one thing, youre going to have to change another to accomodate it..... Hope that helps.
Oh yea, one more thing.. Theres no such thing as a sleeper grand national... there was only ever one trim level that came with the GN's.. It was called "fast as fuk"... There's no chance in hell of hiding that, unless you changed the wheels out to the stock fake wire rims, painted the car sky blue, and let some small rust spots start to form on the doors....