Heat mats WILL burn your snake if they aren't controlled via dimmer or thermostat. I have all my heating elements hooked up the the sliding switch dimmers ($20 home depot, ikea, etc... actually last time I was at Ikea they were on sale for $9). You can wire your own for a few $, but I prefer the read wired type
Any substrate can be used with a UTH, however I prefer to use a thin one (towels, newspaper, etc). Some of my reasons are that by the time it makes its way to the surface, it may not be warm enough, or if it is warm enough on the surface, it may be too hot on the glass. Loose substrates tend to encourage many species to burrow down to the glass regardless of if it's too warm.
Some substrates may not be suitable for some species... it is impossible for us to advise you on what to use without knowing. You also need to think of whether you want something that is purely functional or that is also pleasing to the eye.