I don't worry about the paint being a few shades off. IIRC, Duplicolor white primer is a pretty close match for Alpine white. At least it was on my E34. It's matte, of course, but it'll keep the rust away. Rust > touch-up paint. You can do a really nice job for less than $40 in supplies. You're really just prepping for the professional paint job the car will have down the road, right?
I'll give you some painting advice, (remember it's free, I'm no pro):
1. That Permatex rust-prep inhibitor works really well. I usually spray 2-3 coats of that in 15 minute intervals(it's very runny but dries a transparent blackish). Let that dry firmly 24 to 48 hours according to package directions. That's what makes it work. Alternatively, get some phosphoric acid or naval jelly (but they're a PITA).
2. Spray a coat or two of sandable primer. That white primer is perfect for the Alpine White and it's decent stuff. Just spray it about 12-18" away from the car for fairly even coverage. Use very fine wet/dry sandpaper to smooth it out if you like. Wet sanding with fine grade will make it very pretty but requires a lot of time & work.
3. If you find a top coat, give it a try. Lacquer will work but isn't that tough. I've heard great stuff about the two-part epoxies in a single spray can...but they're one-shot deals at >$20 a pop. You can always spray it again, eh? Please post if you try them, I'm interested in seeing the results.