Range extender for iMac and Macbook air wifi

I suspect that any modern range extender will outperform your friend's old AP.  802.11g is limited to 54 Mbps, but real world speed will be around 21 Mbps.  The range on older Wi-Fi devices has usually been pretty poor.

I'll reiterate that installing a wired AP is much better than a wireless AP.  A wireless AP can be up to 50% slower due to the need to relay traffic between the router and devices via Wi-Fi.  If your Internet service is not that fast, then a 50% Wi-Fi speed penalty may be ok (it could still be faster than Internet).

There's another reason to replace your iMac, functioning as a hotspot, with a real router: security.  It sounds like your ISP's modem doesn't have a firewall.  Therefore, your iMac is open to the Internet and can be directly attacked by anyone on the Internet.  While Mac OSX does have some ability to protect itself, including a basic firewall, you should really install a router to partition your home network from the Internet.  It's better to have a router as the front door to your home network than your iMac.

I think you should get a router, first.  Then, supplement it with a range extender if its Wi-Fi signal can't reach the studio.