I second the advice for starting with a basic TENS unit and the sticky pads that come with it. It's a good way of finding out if you like the sensation or not before investing in a high end unit and accessories.
A dual channel unit will allow you to separately control the stimulation of two different accessories or circuits. There are a number of other features that different models have for varying the output patterns and control much like there are for vibrator patterns and control input.
I have an ElectraStim Flick Duo, dual channel controller. I bought it a number of years ago. Definitely not in the budget range. It's the only one I've used but I haven't felt the need to look into anything else; I know some reviews consider it under-powered. I like that it is portable and can be used for hidden public. Honestly I would have to say the "Flick" feature while sounding interesting has been more of a novelty and I've not really used it. Perhaps it doesn't suit my style of play. I've also purchased a number of the ElectraStim branded accessories.
These are the accessories I have used in my play and my comments:
- Sticky Pads - The most versatile accessories, especially if you have a variety of shapes and sizes. They unfortunately need replacing after a number of uses. Stick them just about anywhere.
- ElectraLoop Rubber Cock Rings - Not limited to cocks, can be used in various interesting and creative ways.
- Silicon Vaginal/Anal Probe - Great for internal stimulation and pairs well with an external or partner unipolar connection.
- ElectraStim Lula - Essentially electrified ben-wa balls. This has been okay for extended play with on-going G-spot stimulation, but definitely on the low end of stimulation compared with a vibrator. I would recommend only if you are a fan of ben-wa balls but results may vary.
- ElectraPaddle - A spanking paddle with electrodes. It's okay for creating varied and unexpected sensations by alternating between holding in place and spanking, but for the most part I prefer to use a separate more solid paddle and separate electrical stimulation pads or accessories. I would not buy again.
- Electric Clamps - I have not used these as much as I thought I would. I'm not sure why.
I hope this helps.