OK, First fix and framing are the same and finish nailer and second fix are the same.
First fix means doing all of the ruff wood work that happens in a house IE roofing, wall framing, flooring.
Second fix means doing all of the delicate stuff mouldings, door frames, and general wood work that finishes off a job.
Now a first fix gun leaves quite a dent in the wood but this doesn't matter as generally it isn't seen, these use larger nails around 30mm to 90mm where as the second fix gun uses brads that don't damage the wood (as much) the brads come in 12mm to 64mm but are alot thinner with no head.
As for makes of nail gun, they all have there ups and downs, Pasload are a good gun as long as you look after it, it also requires a Battery (Supplied) gas (Which comes with nails) and nails.
The De-Walt gun works differently, it only uses a battery and a heavy spring, but these (As far as i know) only come in first fix.
Then you can have compressed air, you need no gas or battery but you do need a compressor, the nails are generally cheaper and more easily obtained.
If it is only this job you are going to do with it you might be better off hiring it as the Pas-load gun comes it a around the £400 mark + nails.
Just to throw a spanner in the works, Pas-load do staplers as well which might be better as the staples are less likely to pull through the thin board, something to think about.
17 years in the hire industry, and working with pasload tools.