With adequate time at hand I'd go for a towerized Zorro-II-controller-slingshot solution: decent performance, boot option, expandability, possibly some fast RAM. Fitting a drive - any drive - into the A500 case is tricky yet possible. Alternatively, replace the 500+ with a 2000 (saving you the hassle of towerizing and building a slingshot) or a 1200 (saving the hazzle of just about everything but getting an IDE HDD and adding quite a few niceties).
Apart from the controller and a DIY slingshot you can get smallish SCSI/IDE drives (<2 GB) for free. Parallel options will be slow, not bootable, lower capacity (ZIP) or energy hogs (parnetted PC as file server) - albeit a bit cheaper and less time-consuming.
... and if you decide to stick with the 500+ do get a 1 MB chip RAM expansion card - the best thing about the 500+ is the easy way to get 2 MB chip.
