An Amiga these days really is for the enthusiast. The A500 Plus predated the 1200 only marginally and Commodore already were developing the 1200 and 4000 to fill that gap in the market for expansion through hard drives and the like. The 3000 gives this also along with the 600. The 500 Plus really became a bit of a hybrid and most of the upgrades were cleverly manufactured to cope with the form shape and space limitations. In the day the kit was very expensive and consequently much of it has disappeared. And because it was often difficult to identify what was actually in A500's and Plus models they often got junked.
Therefore internal storage is tricky if you want to fit a similar drive to say the 1200. Most these days fit their 1200s with modern flash drives... I am told.
You can fit a side car to the 500 quite easily. The problem is finding one that works and also has the power supply.
These two links from my website show you both internal and external drives. I do buy 500s simply to crack them open in the hope that as with the Viper I find a rare card.
The 500 Plus also was fitted with an internal barrel style battery which leak. So removing that is important. See the attached again for some intenral shots.
As to mass storage, there really isn't a great need. And really I would suggest that your interest is not likely to stretch beyond a disk box full of games. Most want the upgrades these days to use stuff off the internet. Personally I would use the 500 as a pure games machine and play with the disks. Thats how they were made to operate and seriously, the machine is so slender, one more disk box isn't going to clutter.
Then if you do enjoy playing with an Amiga go get an A1200 and then you can still play all the 500 stuff. But seriously, please, please look after the 500 Plus. And don't hack it about. They may not be rare, but they are valuable to the community.
Check out my website for loads of information on the Amiga.
Here are those links:
http://www.commodore-amiga-retro.com/amiga/amiga_scuzz48.htmhttp://www.commodore-amiga-retro.com/amiga/amiga_scuzz382.htmhttp://www.commodore-amiga-retro.com/amiga/amiga_scuzz105.htmhttp://www.commodore-amiga-retro.com/amiga/amiga_scuzz493.htmHave fun.