You won't miss much buying your hog at 6 months old, by this time the hog should be considered a juvenile being around 3 feet still and eating fuzzy/weaned rats.
I would buy an UTH about 1/3 the size of the bottom of the tub you are going to use for your Hoggie. Since you are planning to use an xmas tree tub my guess would be a large or xl UTH. It would do well for a hotspot, but in tubs I find that it won't do as well for your ambient temp. Try to keep your tub in a fairly warm room.
Using aspen would be fine if you make sure you buy some quality aspen that isn't too dusty. Personally I use cypress because it simply looks more natural and holds humidity amazingly well, perhaps too much in tubs. When placing the cypress I like to air it out a bit to dry it out a bit first.
For your thermometer/hygrometer combo, I like the Accurite types which you cam get from home depot in the gardening section for fairly cheap(~$20).
Hope this helps. Good luck to you and make sure to post pics when you get your Hoggie
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