I will try and answer most of your qu's but no expert but im sure other people will fill in the gaps. But first of all
to RFUK =)
I'm no good with conversions but im thinking a 4ft viv would be big enough for his age.
Its proberly better to use fresh sand each monthly clean and you can get animal safe disenfectants that can be used for cleaning the viv out.
I think it needs to be just a bit higher than 100F, I think it is about 110F in the hot end (please will someone correct me if im wrong). In the wild it does get quite cold during the night, i read (on here) that many people switch everything off - but i would wait for someone to come and correct this.
Heat mats are not really a good idea as beared dragons cant sense heat beneath them and it can cause them to burn them selves - if a heat mat is needed putting it on the side of viv outside might be better.
You can let him outside when its sunny, but when letting him obv. never leave him alone. many people can see it as risky as there is a chance that he could catch some dieases, but i shouldnt see a problem as long as no pestiside as in the garden (again someone correct me if im wrong)
You could bath him once a week?, especially if he has walked in his poo!
I dont know of any age related dieases but im sure someone will have more knowledge than i do
The slates you have put in the viv should trim down the nails with out having to worry about it, if you have to i would advise taking them to a vets or someone that has experiance doing this, as you may cut too far down where a vien is.
it all depends on the rate of growth of the dragon just keep an eye on him and make sure there isnt any left over skin, especially on the tail and toes.
Calcium powder should be dusted on the veg 5 days a week and then nutrobol multivitamin 2 days a week, veg shouold be given everyday dusted.
For crickets you can fed more than 1/2, i would think about 7/8 depend son how interested he is - again if this is not corr3ect then someone will correct me
As you said they could come from anywhere so there is a risk of pestisides. There maybe some sort of of wildlife you could have but im unsure!
Sorry if ive not been the best of help but i hope i have answered most of your questions! Let us know how you get on!
Oh.. and here is a link to a thread telling you how to put pictures on :
Hope this helps
Keep us updated
EDIT: someone has beat me to it