The problem again with the 400 is the heat. I grew with a 400 and just a 15" axial fan blowing directly on my one plant (NOT RECOMMENDED-but it worked for me somehow), with the door open at all times when the light was on. Problem was the smell. Now I have the heat and smell under control, but I had to get the right equipment.

The 250 sounds like a better idea for you... (Not sure about the footprint and if it will reach 3.5', but it should) You will still need to have some kind of active exhaust, but shouldn't have to be anything fancy. Get the hang of that first, then you can get the 400w or 600w or 1000w package if you really get serious.