A 3 foot tank is barely adequate for one animal (half the chaps will kill me for even suggesting its adeqaute), so i wouldnt go getting any more. Musks dont bask much but they still need the UVB so you are probably better going for a strip or compact which is changed every 6 months. An external filter if you can afford it is ideal and a small water heater (24-26c until he's aroud 3 yrs). Musks like to clamber around so you dont want deep water (7-8inches is grand) and enough bits and pieces of wood or whatever cover for him to surface easily and feel secure. You are really better off just keeping the one indoors and if you were to put them out you'd never ever see them again as they dont do the surface display thing :whistling2:and prefer to scrabble around the bottom of the aquarium in the middle of the night when you are trying to sleep:bash:

 

A 3 foot tank is barely adequate for one animal

A 3ft tank is perfectly fine for a musk turtle!

 

Oh no I didnt mean I'd get more for the same tank, I just wondered if there was an actual good starter kit around where I could just get everything rather than have to shop around endlessly.

 

I don't know any good starter kits, I always put my setups together myself - it generally works out much cheaper as well as more suitable

You do get mesh screen lids but I couldn't help on where to get them. You could ask elsewhere on the forums though as people use them for invert tanks, as well as rodents and maybe other things. Only thing to watch is that any mesh you use isn't too fine, as if your UVB bulb is above the mesh then it can filter out a lot of the UVB.

Don't worry about the humidity, your tank should be open topped (or mesh topped as you say) precisely so humidity doesn't build up, as this can be harmful for the turtle as well. So there is no danger to your bulbs. I use clamp lamps on completely open topped tanks and to be honest if you make sure the lamp is put together tightly and mounted so it doesn't wobble then they really are secure. They're sturdy in your hand as well. Of course you could also use a RCD for extra safety

Exo Terra glow light clamp lamps are very good, but there are lots of similar ones too that are fine (look on Surrey Pet Supplies website - best prices), and even metal clamp on desk lamps like below can be used for your basking bulb if you like the look of those clamps better.

You'll be able to get everything you need online if you want, between Zooplus and Surrey Pet Supplies. Both have great prices and good stuff, but SPS have a bigger range of bulbs and lamp holders. If you can go to an external filter straight away, get that from Zooplus

 

Must admit given the adult size of a Musk turtle I would have thought an adequately filtered and looked after 3 foot tank would be fine for your Musk.
And as above keep it clean and well lit and well ventilated and all should be fine.
Screen lids can be bought for 3 ft tanks but they are easy to make.