Welcome to the Lugerforum.
IMHO, Here are MY answers to your questions in the same order:
1. A shooter? Then I would recommend a Mauser manufactured during the 1940's since they are plentiful, fairly nicely finished, made of the best metallurgy, and manufacturing methods available at the time...
2. Anywhere from $450 to about $1200 or more depending upon condition, its collector value (since you specified matching numbers) and whether you purchase it from a veteran (or his heirs), a dealer, a collector, or a scoundrel... But if you are going to be shooting it a lot, I wouldn't be worried about matching numbers... only condition. Reliablity is far more important than matching numbers on a shooter.
3. Any of the ammo you specified MAY be used, but each pistol has it's own likes and dislikes. It is often reported in this forum that the Walmart bulk pack of Winchester functions fine and runs about $11.00 a box of 100.
4. The best aftermarket mags are Mec-gar... reasonable in price and well made. Some like the Triple-K mags but they only hold 7 rounds. Make sure you buy an orginal or VOPO loading and disassembly tool or you will wear out your thumb loading the mags... Don't buy unknown quality repro tools they aren't worth the money.
5. Spare parts? Extractors and ejectors are the most common parts to fail. Probably holdopen springs are third. Toggles are expensive parts but sometimes you can get a good deal from someone parting out an old Luger... VOPO plastic grips with the bullseye on the sides are the cheapest and best grips to use on a shooter... Wolff springs are recommended for a shooter. They come in three strengths. You can guess and buy them separately or save a couple of bucks and by the three-pack of one of each... and that way they can be custom fitted to the loads you decide to use on a regular basis... The recoil spring should be strong enough to ensure reliable loading from the magazine and prevent the toggle from being beat up by hot loads (not recommended), but not so strong as to prevent reliable extraction and ejection, and allowing the holdopen to work when you fire the last shot.
Because of their solid design... Lugers are found to be very accurate, even when the barrel condition is considered to be ugly... <img border="0" title="" alt="[Eek!]" src="eek.gif" />
Not too concerned about "originality" ??? Then I recommend you replace the front sight with one from a P-38 (They fit the same dovetail) because it will give you a square blade sight picture, and then very carefully square the rear notch to match...
You will be very pleased with the results...