i like the dremel 395 series, 37k RPM max with variable speed
. i agree make sure you get a huge accessory pack. one that has plenty of cut-off wheels, preferably re-inforced ones. they waste away quite quickly especially when your first using a dremel.
use cut off wheels at maximum speed for metal, run the wheel for 30 seconds before you use it though. that is to make sure it isn't compromised. the wheels can shatter at high speed and it's not pretty!!!
always cut away less than needs to be cut and then grind/sand into shape. this way you avoid screwing yourself over by cutting away too much.
for plastics use low speed and only brief contact. if you use high speed and/or contact plastic for more than a short time it will melt. this includes perspex/polycarbonate.
cut off wheel work best if you can cut sideways rather than along the surface of metal. if you are doing a long cut, once you have started it try and get the wheel in and pull along sideways. it will save time and money for cut off wheels.
USE SAFETY GLASSES!!! this is a must. fairly high velocity fragments of random things get flung out all the time and can cause a lot of damage to your eyes. also use a face mask if you can as dust from metals and other things can be harmful to your lungs. the worst thing in my experience is the dust from cutting a PCB (printed circuit board). i learned that the hard way so you don't have to
and the number one rule of thumb for case modding...."measure twice, cut once"
. if you make sure of everything you do it will all go well