I disagree with mpkeelee, I know (AS I have done so myself) that you can build really good looking display enclosures for Ball pythons. It is true that you don't see the animal all the time but looking at the plants and set up is also pleasing and the pride in building and maintaining a good environment for you snake is worth a lot. i use tubs too but somehow feel like I am hiding the snakes in the tubs or I am ashamed of them, just me but that is how it feels sometime.

OK so questions.

I have never used tiles, I think that too would be tempted to not adhere them to the enclosure but have them removable. Never forget heating. There are alternatives to UTH methods (flex watt, commercial UTH or cable) but in practice they are doing the same job heating the bottom. Tile is awesome thermal mass but getting it warm and keeping it warm is another issue. Radiant heat panels would work well too but you just brought up the cost a fair bit.

I have used and been very happy with a 1/4 PVC sheet as bottoms. NOT the aerated pvc (expanded) that most commercial groups use but the solid sheet no bubbles in it. It heats better not as much insulation. You could router a slot for it to slide in and be able to pull it out for cleaning. Just a thought. I have been using all plastic enclosures myself.

Most paints these days are safe once cured fully. (not 'spar' varnishes or anything designed for bathrooms or decks they often have mould killers and insecticides added you want to avoid that, spar varnishes are not designed to harden so they never really cure all the way either) Personally I like poly urethanes (colours in plastic) they are a bit more but are as the name suggests plastic when cured.

Curing is a funny job don't do thick coats use small amounts of paint and a few coats than one thick one. (follow the instructions carefully it works better) Once done make sure it is a well ventilated area and it is warm it usually takes 5 days but I'd give it 7 to 10 myself to be fully cured.

I hope this helps.
Alex