Long post coming up.
There's actually no need to play a GBA game to complete later Pokdex(es). There are no Pokemon that aren't available in Gen IV/ V due to the remakes HeartGold & SoulSilver. With the exception of event legendaries, you should be able to get most Pokemon relatively easily. Rayquaza is probably the hardest to get, requiring the Kyogre and Groudon from HG and SS.
I would suggest you get a 3DS if you want to play any future games; you can play up to Gen V games on a normal DS, but Pokemon X & Y will be released only for 3DS, and it's expected that all future games will only be 3DS compatible. You can still play normal 2D games on a 3DS, so that's better worth of your money.
I think the games you want for full Pokedex completion in a Gen V game would probably be:
- HGSS (both, to get Rayquaza); you will also get one of the Hoenn & Kanto starters in each game after beating Red; the remaining starters you will either have to trade for or restart a game
- Platinum (you can get Dialga/ Palkia/ Giratina from 1 version); no need for Diamond or Pearl, because Azelf, Mesprit and Uxie you can get in B2W2
- Black & White (so you can get both Tornadus and Thundurus to get Landorus)
- Black 2 & White 2 (so you can get both Regice and Registeel to get Regigigas)
So 7 games in total. :I
Rather than get double versions, I would advise you to trade with others in order to get a the few remaining Pokemon you need. I think this only works for B2W2 though, since you can swap Regice/ Registeel keys and get the remaining Regi member that you need. Also, while you only get Latias in White 2 and Latios in Black 2, if you have HG & SS you should have both of them already.
There's also a way to get Landorus/ Thundurus/ Tornadus without Black and White, which is through Dream Radar. However, they will have their Hidden Abilities, which is pretty rubbish for Tornadus and Thundurus... if you don't plan on using them in battle, then that's fine.
The fastest way to trade with yourself would be to use another DS. If a friend of yours has a DS, ask if you can borrow it since that will be the easiest way. If you trade often, that might get a bit annoying, so you can get a DS Lite cheap/ second hand, since you'll only be using it for trading anyway.