I would have suggested kipper2k
http://kipper2k.com/amigaforsale/ but it looks like he's still developing memory for the 600. I would email him just the same in case his website is not up to date. I have the A604 with the ACA620EC which gives plenty of memory but probably a little more than what you want to spend I purchased both from versalia
http://www.vesalia.de/ amiga kit also sold these the A604 has been replaced with the A604n the ACA620EC
http://amigakit.leamancomputing.com/catalog/product_info.php?products_id=1102is also an option. I would have preferred the ACA630 but is currently unavailable.
Don't quote me but I seem to remember that a 68020 is recommended as a minimum for WHDLoad. Kipper2k also has a very cool CF adapter for both interior and exterior. Anyway I hope this helps out a little bit.