My A500+ Project

[HyAmi]

Indeed a nice project. I've considered it myself but I ended up with some A4000's.

Anyway, if you want to browse I would recommend to install a 030 accelerator. As mentioned by Carls, a GVP A530 is a nice option. If you're lucky, you might find a Derringer which can hold up to 32Mb fast ram.

There are some NICs for an A500(+) in a sidecar expansion box but they've got two downsides.
- no passthrough for other devices so an external device like a HD sidecar will be difficult to find. So if you'd choose this option and you can't find an external HD with passthrough, you'll have to find an internal HD controller and an internal accelerator.
- very hard to find (I know as the two I sold were the only ones I've ever seen for sale in a very long time)

Another option is the RR-Net network device (from Individual Computers) that connects to a clockport. It would require a clockport adapter and according to Jens Schoenfeld it would be really slow. I don't know if the drivers for the Amiga exist.

And imho you should definately get a 3.1 kickrom.

[HyAmi]

How often do 'good' accelerator cards come over eBay? Or this is usually a very rare deal?

Not too often. I believe I've seen one viper530 the last  three years. Derringers are a little less rare; maybe 3 in the last 2 years. I'd say it must be a sign of God/Buddha/Jawhe/Allah/...* that a Derringer pops up on ebay right now.

* Select or add your preferred deity

[HyAmi]

However if Workbench 3.1 runs very slowly then I will consider buying one

Well, the point is that Workbench 3.1 itself will run quite ok on a relatively unexpanded (read 68000 processor) A500+. Especially when you have a harddisk and several MB fastram. If you want some programs to run nicely, then imho you want an accelerator