You can use plexiglass, however the silicone wont bond well if at all. Glass is just as cheap and very easy to use. Just get a good glass cutting set to cut it. Home depot has pictureframe glass which worked perfect for my 40g sump.
The type of powerheads will depend on the type of corals you keep. If you keep Small polyp stony corals you will want a much higher turnover than large polyp stony corals. Koralia's
are a very well known and used powerhead. They are not to expensive but the one you need will depend on the corals you keep.
You can use metal halides or T5HO. T5 will allow mostly any coral although some may need to be placed higher up. The nova extreme
fixtures are a common recomendation.
For a skimmer, Always go bigger than you need, incase of overfeeding or upgrading. Octopus skimmers
As far as heaters go, you want 4-5Watts per gallon on average. This depends on the room temperature. I would suggest 2X150W heaters.