SSD's usually come in 2.5" form factor so you'll likely need an adapter unless you use a laptop or small form factor pc (does a mac mini use 2.5" or normal full size hard drives...), adapters sometimes come with the ssd but they're only a couple of quid if it isn't supplied.
I personally use a marvel based (the chipset to pick in most people's opinion) crucial m4 128gb, cost me about 80 quid, it's quoted stats aren't quite as high as say the sandforce based chips but it has 2 of the more important features - stability and decent read when the drive is loaded with software etc, sandforce (ie ocz agility 3 etc) lose performance when loaded and they've also had some issues with firmware. Also in the real world you will not notice any of the difference quoted by the benchmarks. If there's one you like, let me know and I'll say if it's any good or not
As to the external drives - I'd suggest getting a hard drive housing that can support mirrored raid (aka raid 1) or raid 5. This requires a minimum of 2 drives for raid 1 and 3 (I'd recommend 4/5 though) for raid 5 but gives added piece of mind if one of the drives fail.
Alternatively you could have more than one external drive and sync them up.
Now the heat is unlikely to be hard drive related, my pc's are gaining around 10 degrees (room is not nice) during the heatwave... there's only so much you can do to reduce the temps.
The options you have are:
- Stick it in a fridge - not recommended
- improve the cooling - better fans (I use a fan controller to speed mine up during hotter weather), better cpu cooler, better thermal paste etc - easy to do on a windows pc, not so easy on a mac though... also can take time to do
- move to the north pole
- get air con for your room (can be had for about 200 quid)
- dust out the pc case, dust is a major contributor to pc failure due to making things overheat - get one of the air cans, non flammable obviously and spray into the vents on laptops while it's turned off. You'll be surprised by the amount of dust in there
- make sure you have clear air around the pc
- and as daft as this sounds - don't open your windows, just close all your curtains so the sun and heat can't get in. It can actually make your room cooler because you're not letting the heat get in in the first place. Silver window 'privacy' film can help keep the heat out too.
the ones in red are probably the easiest option to try first.