If your building it from scratch get the concrete floor floated it will come up smooth as a baby's pollybond and paint with an industrial floor paint, no the one in B & Q its the cheapest way. Carpet or floor tiles will break your heart if your planning to do some real work in there. Welding grinding, spilled oil or water will turn them into a soggy oily burnt mess in no time. Like the idea of the rubber belt cut to size.
If it's going to be used as a workshop rather than solely as a garage where things are stored then good quality floor paint backed up by a couple of rubber backed industrial floor mats that you can roll up and move out of the way if you're doing 'hot work'. Decent mats can be washed with a detergent then hosed down and oil just floats off them so they're good as new plus they're a lot better to kneel down on than plain old concrete. Sometimes it's handy working in engineering because I've got a cut down floor mat that I use as an 'environmental protection sheet' (I shit you not) when I'm refueling my KTM during off road races.
I used this rubber floor mat roll from Machine mart in mine. Its very hard wearing and i have had no issues with it when its damp. Rubber Matting - Studded - 3mm - 1000mm x 10m - Black - Machine Mart
I dont know why my photos are always the wrong way up! and yes, the confederate flag was a 'souvenir' from my time in the Southern states, I bought it naively when I didnt really understand what it stood for!