When you say $500 amp, do you mean a home theater receiver? If so, using its amps it is not going to do justice to the Concertino, although if he is just listening to mp3 music, it may not matter a lot. If these are the old Concertino bookshelves, they sold for $1K each. The new Domus floorstanders are even more.
If you just want an amp, then the ATI 150x is in your price range and has plenty of power.
Following on to what Mceljo said, in oder to drive the HT and the whole whole house from one receiver you need a 5.1/7.1 receiver with zone 2 capabilities. One with pre-outs will allow you to incorporate externals amps for the LRC and whole house.
One approach would be a receiver with pre-outs and zone 2 capabilities and an ATI 1505 amp. You could use 3 of the 5 channels on the ATI for LRC and 2 for whole house. The LRC would use the pre-outs of the receiver and the whole house would use zone 2 pre-outs. The surround speakers could then be driven from the receiver, as they are not as critical. Zone 2 lets you play whole house music separately from the HTsetup.
Hopefully the 3 sets of whole house speakers have impedence matching volume controls. If not, you probably need a impedence matching box. If so, those are mostly 4 ohm, so you need an amp that can drive 4 ohms.
Used receivers are OK, but you probably want one with HDMI inputs if he is every thinking of Blu-Ray.
Note that zone 2 typically requires analog inputs (receivers in this range can only decode one signal at a time). So, to use his iMAC with zone 2 he is going to have to run an analog cable from the iMAC to the receiver or have a separate DAC. He can use the headphone out jack (assuming is has one) although that means using the DAC in the iMac. Otherwise he needs a separate DAC.
Probably more than you wanted, but hopefully some of it is useful.