One of the Reasons I Don't have a Sump

[BCOrchidGuy]

If you're going to put a sheet of Ply under your tank go the extra step (and it's cheap) and put a sheet of foam insulation down between the tank and the ply. The foam will support the tanks actual glass bottom and if the ply warps or does anything else funny the foam will absorb it.
The heater issue is why some people like to use two smaller heaters rather than one that's rated for the tank, IE rather than a 250 use two 150s. Most heaters are recommended on the idea that you need to raise the tank temperature by (I think) 5-10 degrees, so they recommend 5 watts/gallon. This means a 5 watt heater that's rated for a 10 gallon tank shouldn't work on a 27 gallon tank but as long as the air is room temp and the tank isn't a discus tank, the 50 will work nicely. I used a 250 on my 150 gallon and it was great. I've got a 10 gallon tank I'm using as an incubator in another room and a 25 watt heater is holding the water in there at 95+ degrees.

Doug