Sorry I didn't answer any of your questions......
Start with the easy stuff..... Valver is quoted as I.g 12 and the Williams is I.g 16/17 depending on which insurance company you use and where you look.
The mpg I get out of mine varies from very low 20's to mid 30's, obviously the latter is on the m-way.
There are a million one things to look for as I am sure you will appreciate being a 10+year old car.... but in general i would inspect.... Arches for any signs of rust and rear 1/4 pannel where the bumper hits the arch as this is a VERY common problem.
Sun roofs always leak, ALWAYS.
Steering columns are generally a little lax and again is a common problem so if there is a little play do not worry about it.
As for the engine, its v difficult to say, but upon cold tickover it should be around 4-6 bar (on the dial) and be at a solid 6 while driving for about 5 minutes. Mine used to be at 6 when cold but depending on oil and other factors mine will generally sit around 4ish now.
When cold it is normal for it to be tappety as there old, however any noises should soon disapear when warm, mine do, and if they didnt i would be worried.
The normal operating temp whilst running is about 80dc, mine will go to 90/100 when doing 80/90 on the m-way/booting it (never seen it above 100 apart from once in the summer). Also when booting it at what ever temp the oil pressure is still around a good 4bar. When on cold tick over oil pressure can be anybetween 1 and 0.5.
The water temp usually also sits at the first major line, when inspecting let it run to make sure the fan kicks in.... the fan kicks in at the 2nd major line btw.
The valver has a bhp of 137 and the williams is 150. If i was in your position i would look at williams only as they will probably have been better looked after (being a drivers/ltd run) as well as the fact it will hold its value more.
You will also be able to see that most valver owners replace their 1.8's with a williams/megane 2.0 .... go figure.
For performances check out williams clio or take a look at the 1/4 mile times.
Hope this helps.