You may be better off as suggested just getting a second hand A1200, but even then I recommend the following upgrades:
1) 3.1 ROM (2 chips for A1200; 1 for A600)
2) IDEFix IDE adapter board (for using up to four external or internal 3.5" drives on two channels). You may even use an internal IDE to compact flash adapter instead of a harddisk for booting in under 3 seconds.
3) 68030 or better cpu upgrade (there are many for A1200; Blizzard, Microbotics and DKB Cobra/Mongoose allow up to 128MB fast ram; there are a few *rare* 030s for the A600 which clip onto the 68000 cpu and Jens Schoenfeld may release his new modern 68030 board sometime). Some A1200 68030+ boards also offer SCSI functionality.
4) PCMCIA network card (RJ45 or wireless using Prism2 chipset), make sure you get a 16-bit Type I/II card.
5) Internal or external scandoubler (to use large modern flat screen or standard SVGA monitor); I personally think the external ones may be better because they generate a lot of heat and the internal ones are clip-ons so cooling may be an issue.
6) Clockport expansion: depends upon what you want to do, but there are 16-bit soundcards, a USB card (Subway), and fast serial/parallel adapters available for this little header next to the ROM chips on an A1200 and available via either a Gayle clip-on board or new 1MB chipram board for the A600 (both available from Individual Computers/Jens Schoenfeld).