Yes, Exxos has a lot of nice add-ons!
And while you're at it, with such an old computer you should really recap (replace the old capacitors) the power supply. Exxos sells recapping kits for the various Atari power supplies
. Of course this means you (or someone you know) needs to have basic soldering/desoldering skills and know what you're doing (in the case of the power supply, to avoid electrocution among other things). Depending on the other upgrades you may/may not need to do a little soldering.
Another suggestion is to get yourself a MonSTer board
This multi-function expansion board adds two IDE ports so you can attach an IDE to Compact Flash adapter (cheaply bought on eBay) which means you can use a Compact Flash memory card as a silent and fast hard drive replacement! And all within the computer. There's a looong discussion thread about it here
with more detailed info.
It also allows for changing the TOS (Atari's operating system) in software and if I'm not mistaken you can change between two different types of operating systems if you like, or just upgrade to a newer TOS without reprogramming the EPROM chips, which of course is more convenient.
The Monster also allows you to attach a realtime battery-backed clock board (also available on the cheap through eBay) which is sadly missing originally on the Atari ST.
Finally there's an additional 8MB of RAM, but this ALT-RAM, so I suppose you'd still need to upgrade your normal ST-RAM as suggested earlier in the thread. ALT-RAM can be used for a lot of software, but not all (correct me if I'm wrong anyone), and you have to tell the software to use this memory beforehand.
I suppose you'll be downloading software from the different Atari websites/FTP sites to try out on your STFM, in which case transferring them via a floppy disk is the usual method to move the files from a PC/Mac over to the Atari. Another method is to use a USB memory stick with the help of a Unicorn
which is a small box which plugs into the hard disk connector on the back of the Atari.