On the whole, I think she'd be happier in the larger viv. At 3' it's a little large for her, but if you provide lots of cover and places to hide I wouldn't be too worried. If she doesn't adjust to the larger viv within a few days you've always got the option to mover her back.

I wouldn't replace the lid with mesh, as long as the viv has suitable vents you should have enough ventilation for her. In the UK I think you'll find it more of a struggle to keep the humidity up around 50% - I'm making an assumption that you don't live in a particularly damp house. We use ExoTerra vivs for our snakes at the moment, and I end up covering most of the mesh lid to reduce the ventilation and raise the humidity.
As far as cables are concerned, if the viv doesn't already have holes for thermometer/hygrometer/thermostat cables... it's easier to drill and seal the wood, as you just need to use a putty to fill the hole around the cable (or hot glue, or some other filler). The mesh will be harder to seal without resorting to tape - tape is a bad thing in a viv.

Check the gap between the two glass door panels. A small garter can often squeeze out through that gap, and believe me, if there's a gap a garter will find it. There's a few techniques for closing the gap if you need to do that.

Other than the above, anything in particular you wanted advice about?