I am making the case for the light engine out of MDF to be used with a 150W lamp. It will have a cooled with an 80mm 240V AC fan ...

I am going to shield the inside with some sheet metal... but am I pushing my luck ?

I think it sounds reasonable, as long as you shield it properly(you could even use some heat resistant paint-stove paint) and it should be fine.
i wouldn't have the fan blowing on the bulb though as have heard this can cause problems. instead put some vents in the light box and have the fan extract the heat.

good luck

That's certainly a workable idea but one must always keep in mind the potential hazards involved. Monitor the completed project extremely diligently during the first hours of operation.

I don't think so. If you haven't used a 150w lamp before, I think you'll be surprised at how little heat it produces. If you're shielding the inside, you should have no problem at all. But, as everybody else noted, do a lot of testing before you put it into operation full time.

My first projector uses a 400 watt bulb. The sides are made of pine and the top and bottom are hardboard (Haas style). I encased the the light chamber in stainless steel sheeting. I have two fans rated at 90 cfm to suck hot air out the back. The only problem I have with heat is the room temperature is raised a few degrees. Thats why I am going to rebuild with 150 watt bulb. As long as you're careful you shouldn't have any trouble.