Optoma HD70 Lamp Life and Replacement
Lamp life is rated 2000 hours at full power, and 3000 hours in Eco-mode (low power). With ImageAI engaged, the lamp life should be similar to full power. The lamp door is on the bottom of the projector and if you have ceiling mounted your HD70, you'll have to unmount the projector to change the lamp.
HD70 Projector Screen Recommendations
Since the HD70's black levels are good, but not exceptional, a light gray high contrast surface is a good choice - the usual suspects would work: Elite's HC Gray surface, Da-lite's Da-Mat, and of course the Stewart Firehawk (which I use), as well as comparable surfaces from other manufacturers.
If, however you need maximum brightness, and don't mind the missing the slight improvement in black levels that those screen surfaces provide, consider the affordable Carada Brilliant White, Da-lite Cinema Vision, and the Stewart Studiotek. If your room is long and narrow, you could even go with a higher gain screen such as the Optoma Grayfox (I haven't worked with that screen) with its 1.8 gain. It is sure to hot spot a bit due to the high gain, but if you are sitting near or dead center, it will give you more brightness.
Screen size, is more of an issue. To watch in best - Cinema - mode, with the lamp at full power, I wouldn't recommend anything more than 106" maybe 110" diagonal, unless you go with one of the high gain screens. The last movie I watched was Flux Aeon, which has excellent production qualities, and I found that, on my Firehawk (in a room with light walls, but almost no ambient light) that I wasn't happy until I had the image size down to right around 106" diagonal. It certainly was way too dark if I tried to fill all 128".
Consider then, if you want to save money by running in low power, or just don't like the higher fan noise of the ImageAI or full power modes, then I suggest no more than 100" diagonal. (The measured difference, if you recall, is low power is 20% dimmer than full power. And, in terms of screens, that's roughly the difference between a 110" and a 100" screen.
For non-movie viewing with more ambient light, remember the HD70 has a very respectable amount of lumens, so with a 92" to 106" - even 110", your projector will be able to handle modest ambient light without difficulty.