Ok, to start the most important things are uv and temperature gradient so ensure that you have allowed for enough space to create the gradient and you have the best equipment available. So we're talking a 4x2x2 or larger vivarium and the Arcadia T5. I understand you haven't asked this and I'm hoping you know it already but its the most important place to start. This should all be set up so you have the temps correct and steady before purchasing your dragon. The last thing you want to be doing is messing around with heating, changing bulbs, adjusting temps, etc, etc while your beardy is trying to settle in to his new home and see you not as a threat.
Ok, now to answer your questions. Firstly, no to the heatmat. They can be dangerous and quite frankly useless for BD's. they do not increase ambient temps so will only be effective if your dragon is lying on top of it, dragons don't detect heat from below all to well so may not recognise if the mat has malfunctioned, overheating or generating a hotspot. This can result in a burnt bearded. Basically, its a high risk for minimal reward. During the day you shouldn't need any other heat other than your basking bulb and at night you should have no heating or lighting at all. A temp above 10c is fine for night temps, if anything you want to keep it lower than 20c too.
Calci sand is a terrible substrate at any age! The calcium can attract the BD's to eat the sand and at high levels of ingestion it can cause impaction especially if your husbandry is not spot on and/or your dragon is dehydrated. For babies your better off using no loose substrate as they may have a poor aim on food and generally be clumsy ingesting any loose substrate. I would recommend tiles, Lino or paper towel, in that order.
Water bowl will be fine in there constantly as long as it is not too large and is always kept in the cool end.
The best disinfectant I find is F10, brilliant stuff and lasts for ages.
Lastly, crickets. Oh the dreaded spawn of satan. They're the worst, most annoying, fiddly, smelly little
's! I personally wouldn't bother and would suggest ordering locusts in bulk online. It will only work out a little more expensive this way but totally worth it. Crickets get everywhere, they're a bugger to catch and you will spend a good hour removing them all from a viv before lights out, which, if you don't do could result in a nibbled at beardy and some sore bites which may lead to infection. You won't have this issue with locusts, plus they're much easier to catch. If you do want to try crickets, I'd recommend using tweezers and feed one at a time. Place it directly in front of your BD so he has a chance to catch it before running away. You could also try putting them in a lidless container that they can't escape from but can be eaten from.
Hope that helps,