Drilling the tank or having it drilled will depend on the glass itself, if it's a new AGA you might be able to find out if any of the glass is tempered. You don't want to see the results of trying to drill tempered glass, it's not pretty......

Think the 175w over the 40g tank should be just fine as far as light output, with a higher end reflector it would be great, I don't think you'd burn anything, a 250 with a wider dispersion reflector would likely be just fine too. Anything over 250 I'd be more concerned about heat.