worst case or one I prefer to avoid is buy cheap speakers and then decide get an amp and then speakers have be replaced. I much prefer underdriven speakers then getting an Amp down the road. Generally speakers are were sink most of your money besides HU and I buy up as much as possible. Only other issues are what speakers can fit in Mustang. Generally 5 1/4 but there is also depth issue to consider as well. I heard people putting in 6 1/2 speakers in mustang and they generally get better Frequency response. Not to mention better deals can be found on 6 1/2 then 5 1/4. The 6 1/2 Focal I mentioned earlier are some best speaker for under 1K and on Crutchfield staff's picks.
2011 GT Coupe Premium | MT6 | 3.73's | Security Pkg | BREMBO | Comfort
Installed JLT oil catch can, Tint, Beltronics RX45/shifters