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Gooday George,
Welcome to ISS, sounds like you have the perfect spot for enbarking on this wonderful hobby.
First up the golden rule for amatuer astronomy - The best telescope is the one that gets you outside regularly to use it.
There is no point buying a heavy 16in scope if you can't move in and out of Garage etc. You can't rely on the wife to help you carry it out each time she'll end up despising your new hobby.
You could put it on wheels and run the risk of it being unstable. Plus you will have to learn to love balancing your body on a step ladder in the dark to reach the eyepiece.
However if you are serious about the hobby, build yourself an observatory, an 3x2 Metre metal shed on a concrete slab, that can be jacked up and placed on wheels to roll away is the simplest.
This is what I did and only took about 3 weekends worth of work, I just laid some pavers behind the shed for the wheels to roll onto, cut a new door frame at the front of the shed, for the telescope to pass through, then hired some axial props to jack the shed up and bolt the wheels (castors) underneath attached to a 3x2 inch wooden frame.
If you are prepared to do this then go for the 16 inch, otherwise if you are content an carrying it out each night, get the 12 inch.
As far as brands go (Meade, Lightbridge, Orion, Skywatcher etc) they are all good and only differ in some features, like mirror type, focuser provided, whether or not they can track the stars or electronically GOTO to a chosen destination.
Don't worry about Pyrex or BK7, yes Pyrex does cool down quicker but also attracts dew quicker IMO. This cool down time is only relevant for maintaining a stable focus, if you have to turn the focus knob a few more times with BK7 than with Pyrex, it doesn't really matter.
By the way Dobsonians are a general purpose instrument, great for nebula's and galaxies, and average on planets, the best telescopes for planets are long focal length refractors.
For your friends question, a 6mm eyepiece will give more magnification but is really only suitable for steady seeing, otherwise stars will be bouncing around in the eyepiece. I hardly ever use my 6mm and only go down to the 12mm eyepiece because of seeing issues, I mostly use the 25mm.
Regards
Bill
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