Welcome to TBHP ICE, Harry!
You'll fond most of what you want to know here:
http://www.team-bhp.com/forum/ask-gu...io-thread.html (The Swift Audio Thread)
AND here:
http://www.team-bhp.com/forum/ask-gu...k-25k-30k.html (basic ICE - systems at various budgets -10K, 15K, 20K, 25K & 30K)
You'll find good HU recommendations (similar to your requirements) in this thread:
http://www.team-bhp.com/forum/ask-gu...ensive-hu.html (Budget install with a expensive HU)
The Swift has provision for 6 to 6.5" speakers in the front doors & rear doors. You can get ovals (6x9") installed on an MDF parcel-tray if you want to skip using the rear doors, or if you want 6x9s.
For 15 to 20K:
HU: 8K-10K (JVC/Sony/Kenwood/Pio7950: since you need a HU with USB input with speakers within this budget. Most HUs above 6K, esp. Pios will have aux-in implemented one way or the other).
Fronts: 5K (JBL CS2165c 6.5" 2-way comps)
Rears: 3K (JBL2165 6.5" coaxials)
Getting a sub & amp also within 20K will be tough and you'll have to compromise for not-so-good equipment all through. Adding a Sub and Amp will also involve at least 2K additional for power & RCA wiring.
I'd suggest getting 6.5" speakers for the rear for now and doing the amp & sub later. Or you can skip the rear speakers now and get an amp & sub instead by stretching the budget just bit.
Sub: Pio306C (about 3K)
Amp: Pio 6200 (about 7K) or the Blau GTA special (6k) mentioned in the 3rd link I posted above. You can use these amps to run your front components & the sub.