My advice is to either use th 2kw amp and enjoy it at reasonable level, or do the alternator and a stack of batteries, etc.
Doing some interim aproach and upgrading alter (when you have more money) will just gobble up more money.
A 2kW amp is not a week amplifier... it should be more than enough for most people not competing for ear bleeding power.
Or save up if you feel you really want the whole 3500w.
Of course ^this^ is all just opinion...