Looking great! I'm going to go ahead and place my concerns for the wheels as well. But if it can hold your body weight then that's a good start. By the looks of it, worst case the wheel will break, but it doesn't look like anything would get damaged. At least not badly.
When getting my Castor's I did a couple of weight calculations:
A single 2440x1200x18mm sheet of MDF weighs approx 30-40kg. I bought 3 sheets, so at max, 120kg. (worst case of course, because there's a lot of scrap). And a TV weighs approx 40kg. So MDF + TV, worst case, is around 160kg. So I bought 4 Castor's rated at 80kg each.
Question. Are you gluing or just screwing? It's deceptive, but glue on bare MDF to bare MDF is way stronger than screws. I've gone overkill on screws only to find that the glue is stronger anyway. The only real plus for the screws is to help "clamp" whatever piece while the glue is drying.