600$ is 460Euro atm, those funds are insufficient for even buying the basic (and quite lowend) SAM440, which go for about 585Euro (listed but out of stock at vesalia.de). You should be able to get it without VAT (as your outside the EU) bringing it down to 487Euro, but thats just the bare board, no RAM, no HD, no case, no P&P.
Used Pegasos2 in the G4 variant are a much sought after item and will easily fetch more than 600$ (for a complete system). The 600MHz G3 version should stay within in your budget even as a full system (but they seem even rarer than the G4s).
Pegasos1 (also G3/600MHz) can be had at bargain prices, but the HW has a few quirks and other options (see below) offer better value for money.
Another option is the Efika, which still can be bought new (old stock but with full warranty). Best option for you would be US-reseller Directron :
http://search.directron.us/newsearch.php?find=efikaBut you should understand that the Efika is aimed at markets were low power and small footprint are more important than massive computional power. You can build a 100% silent (no moving parts) computer out of it (if you add and Flash-based HD) but don't expect it to replace your major desktop PC.
Another options are used PPC-Macs, the current options are:
Any G4 based MacMini (the 1.5GHz version is to be preffered due to it's increaesed VMEM).
eMacs (only the 1.25GHz has been tested) wich can be bought for as little as 12Euro !!).
Support for G4-PowerMacs should arrive shortly, PowerMac-G5 and PowerBooks are being worked on.
Confused enough ?
Now we go to SW:
Their is either OS4, which has the Amiga name&sticker, costs about 90Euro for the Pegasos2 and is included in the price of the SAM440 (it also supports some discontinued boards by "Eyetech" but these were of questionable reliability and performance).
On the Pegasos1, Pegasos2 you can run the 100% free version 1.4x of MorphOS.
On Pegasos1, Pegasos2, Efika and the listed Macs you can run MorphOS2.x which costs 111Euro atm (you might get lucky and find a used Pegasos1/2 or Efika with this allready included). A free trial version can downloaded at
http://morphos.de/ , this is 100% identical to the bought version, only that it will slow down after 30min (counting from last reboot).
So easiest and cheapest way to take a sneak into the community is to get a used Mac from eBay, install the free MorphOS trial and than decide if you want to spend more money.