Hi,
The 2 things i would look for and ensure your one has (whichever brand), are 1) a light on it, sounds silly, but it makes it SOOOO much easier to spot those little hairs that otherwise you would miss and only spot later on!
2)at least two variations of speed and changeable head with diffferent numbers of rotating blades. Very useful for when you first start using it and you arent used to the sensation, Start on slow speed and with fewer blades, then try it faster, then switch to the full number and higher speed.
Also worth knowing, cooling your skin beforehand (some of them advertise various "ice" functions, WILL help numb but very temporarily, and actually makes it harder for hairs to come out as the follicles contract. Not really worth doing.
You are better off having a lovely hot shower or bath to soak first, which opens the follicles, and taking a painkiller (pref one with ibuprofen) about 30 minutes before you start.
Good luck. I wouldnt go back to shaving or waxing from my epilator now- i even do my underarms!!