Shouldn't be any problem in changing your shoes and in any case it's got to be better than running a marathon in something that's going to cripple your feet.
Your experiences with the Creations are interesting. I had a pair until recently and initially they were really good - superb cushioning and pretty supportive. But they only lasted about 260 miles and towards the end I definitely felt the support was giving way and I was getting quite serious blistering under my arches, which I've never had with any shoe before.
Chucked 'em in the bin and I'm afraid I won't be buying any more - I expect about 500 miles out of a pair of shoes, particularly when they cost 90 quid!
Yup, I've posted about my own experiences with the SDM on other threads. It is very accurate and repeatable but I did have some teething troubles getting it working when I first got it. That said, other folks have posted about their experiences and don't seem to have had any problems at all.
One thing to point out, though - the business end of the SDM is a footpod that you tie into the laces of one your shoes. That footpod is, according to the instructions, "splash resistant". I've used it successfully running through puddles and in heavy rain but I don't think it'd cope with being completely submerged in mud or water for long.
I've had an SDM for about a year now. I'd have concerns about whether or not the footpod would hold up under the conditions you mention. It's certainly puddle and (heavy!) rain proof but I don't know about it being completely submerged for any length of time.
... Just had a look in the instruction booklet that comes with it (what, read the manual?! ;-) and it describes the footpod as "splash resistant".
If you do mainly or exclusively off-road-in-the-mud running, I think you'd probably get better use out of the Timex. It uses a GPS unit that you can strap to your arm out of the way of the wet.
Rather than trying to set a weight target based on your running, I'd be inclined to set one based on the healthy weight range for your height and build.
If you go to something like Yahoo! and search for "Body Mass Index" or BMI, you should be able to pull up some info on BMI, which is a recognised way of determining healthy weight ranges for your height.
If you can't find anything suitable, let me know and I'll send you some info.
I have personal experience of this - about 2 years ago I was *way* overweight and couldn't run upstairs without getting out of breath. I now compete, on road and track, for the local club and am having a great time with the running having lost 4-and-a-half stone to get back to my ideal weight.
One hint, though: Don't obsess on the scales! It's easy to do that.
One other thought just struck me - you could talk to your GP and they should be able to recommend a suitable weight?