Here are a few tips:
Take inventory of your chips on your motherboard. Find out the Buster, DMAC, Ramsey, and SCSI. You still can find Buster version 11's and I would recommend you upgrade if you don't have it. The DMAC 04 and Ramsey 07 are near impossible to find and upgrade. If you plan to upgrade to a Commodore 3640, be careful since some DMAC 02's had trouble with that card.
IMHO, you will need 16 total fast memory if you plan to surf the web. If you want to upgrade the zip memory, make sure to see what kind of chips you already have. If you have 80ns static column chips, try to match them when you buy new memory. If you mix page and static you will have fun playing with them to get them to work.
If you plan on really using this system and don't mind putting money into it:
1. There are only 2 (hopefully 3 someday with the ultimate ppc) accelerators that you can use that have local memory and a faster scsi controller. The Warp Engine 3040 or the Cyberstorm MKIII. You can get away with the mercury 040 or the GVP if you don't care about faster Disk IO. These cards will really make a huge difference in performance and usability of the 3k. I do have a Commodore 3040 in a 3000 now and it works fine, but it is only 4x the speed a stock 3k. There are more options for other cards, but you will have to hack the 3000D case to get them to fit and work properly.
2. Add a video card. These are easier to come by. jammarob has a GVP spectrum which is a nice card. It has a path through with is very handy. You won't need the indivision ECS with the video card, get a video card and be done with it. IMHO the Amiga video has 3 tiers of video cards (speaking only for zorro cards). The Picasso IV, Cybervision 64 (or 3d), and the Retina BLT Z3. These 3 cards have the fastest chips and offer the best performance. The second tier: GVP Spectrum and Picasso II are really fine cards both provide pass through and have decent chips on them. The 3rd tier : The Retina ZII and the Merlin, they are ok but wouldn't be as nice as the others.
If you are going to hold off on the accelerator for now, hook the 68882 directly to the 50MHz crystal (see aminet for instructions), this will boost the math and give a cheap speedup.
If you have green corrosion from a leaky battery from the past, I would recommend clean the MB. May as well reseat the chips while your at it.
As far as where to purchase Amiga stuff: ebay or amibay are your best bets...
Have Fun!