would i want 4 or 8 ohm subs and do i need a whole extra amp for them or just run the infinity's and the subs off the 4 chanel and n leave the rear ones hooked up to the headunit
You can run the rears off the head unit
Front Infinity's off the front two amp channels.
You could use a separate class d amp for subs ... assuming you use two subs, if it's rated at 2 ohms, you need 2 - four ohm subs. You would be able to up your power with separate amps, but it will also increase cost.
If it's a four channel amp, rated for 2 ohms, you can bridge the two rear channels and run 2 - eight ohm subs (total ohm speaker load would be 4 ohms, and because the amp is bridged, each channel "sees" 2 ohms).
2 - four ohm subs wire in Parallel = 2 ohms
2 - eight ohm subs = 4 ohms.
What you will have to figure out in your shopping travels is 1) total ohm load of subs, and 2) The lowest ohm load the amp you are looking at can handle.
Also, don't count out 5- channel amps if you can find a deal.
A good place to find discount and refurbished amps is ikesound.com ... search viper, orion, dei, ppi, and refurb. - they had a directed/DEI 5-channel 650 watter listed for $100.00 Also, webzone had some refurb kenwoods, I don't know how good a deal they are though. I'm also sure there are a lot of good deals you could find across the web.