So firstly what do people consider when choosing a substrate, and what do you use?
There's really three main choices. 1. Bare bottom 2. Normal/shallow sand bed 3. Deep Sand Bed (DSB)
1. You can keep everything clean easily, crank up the flow to stop detritus and keep SPS happy, however, it's not as attractive (IMHO).
2. What most people do. An inch or two of sand (not more). Don't go for crushed coral, as it's very coarse and can damage the mouths of sand sifters, and collect a lot of detritus.
3. DSBs need research and special care (I'd advise one of the first two unless you research and stick ridgidly to the DSB guidelines).
Secondly quite set on going with a red sea 250, are they as good as they seem?
Yep, they're good. Lots of members have them, or other size variations of them.
Lastly, for now , would a single for tech mp40qdw be enough, or better to have 2x mp10qfw's ?
Either option gives you enough flow, but I'd go with two, as you're more likely to get rid of dead spots, and if one fails, you have something providing flow, rather than nothing.