When I did mine, I really couldn't be arsed to change the wiring ( although I knew I should have) but the sound I get from the HU is greatly improved. I think the sub works ok. There is a lot of bass.
Keep checking on eBay but for a double din you will be looking at at least 350 fitted in the car. As u will need the adapters and the fascia plate.
I was very lucky in getting my HU off the forum for quite cheap and getting the adapters for a good price. If it weren't for the forum I would have paid about 350 for the HU and 50 for the loom adapters so all in fitted 450.
Just keep your eyes peeled on eBay as there are always bargains. Also a single Din unit would be much cheaper.