Projector Buying Advice for a Newbie

I've just finished my 7.1 setup and I'm now planing for my next big purchase, which will be an HD projector, in a couple of months time.

Now, I will be first to admit that while I've picked up a fairly good working knowledge of AV equipment over the past 10 years, projectors are the one area that have completely evaded me.  However I have used a lot of projectors when giving presentations at work and I know them to be awkward, hot, noisey EDITEDs.  You can never get the picture the right size or shape or alignment, colours either look washed out or so contrasting it gives you a head-ache  - now obviously I don't want this in my Home Cinema!

So we're dealing with light projected through a lens... I can guess the critial parameters are:

 - The distance the projector has to project over (I think this is called the 'throw' right?)

 - The size I want the resultant image on the screen.

 - The brightness and contrast ratio (as this will replace my TV and potentially be used during the day, I think this will be critical).

 - The screen, which will be painted onto the wall, at first with good old white satin paint, then perhaps with some special screen paint when I deem it a worthwhile purchase.

 - Position - it will be wall mounted on a shelf, directly behind the main seating position.  I can't ceiling mount because my ceilings are 5 meters high!

A few no-brainers, it will be 1080p in 16:9 format, with HDMI connectivity.

So the distances are the easy bit, my projector will have to throw over a distance of 4.3 meters, I want the resultant image to be 2m wide (it has to be to fit between my wall-mounted speakers) - so at 16:9 this will be 1.125m high, and 2.29m diagonally.  Now... once we start talking about lens optics, thats where my knowledge stops.  so:

Question 1: How can I be sure that the projector I am buying will be able to project the image size I want over that distance?   I at that throw distance I am more concerned about it being too big than too small!

The other thing is the brightness, which appears to be measures in Lumens and seems to vary betwen about 1000 and 10000, mostly dependent on how much money you spend!  SO this will obviously need to factor in the size of the room, but also the throw, as the light will attenuate as it dispereses (I remember that bot of physics class!). so:

Question 2: How do I decide what brightness (lumens) to get?

and two more quick questions:

Question 3: How does conrast ratio fit into the equation?

Question 4: What is a 'good' lamp-life now-a-days and, as with spares for anything, are lamps standard items or do you run the risk of your lamps being discontinued long before your projector needs replacing?

Thanks as always