Scalextric in garden shed! Plastic or wood! Any suggestions?

Wood offers better insulation but go more than 40mm thick if you can. Allow space all around it so you can re-coat it with preservative every 5-10 years.

Plastic will succumb to UV degradation and will probably be as expensive as wood. Plus its more difficult to line it and more difficult to screw track table supports to it...

I would add that I have a galvanised steel shed 8 feet x 13 feet and it was much cheaper than wood or plastic and it has lasted 10 years so far with no visible corrosion BUT it would need lining for use with a track - if I hadnt converted my garage into. Double-skinned man cave I would have gone with a wooden shed with 47-50mm thick walls and 100mm of polystyrene floor insulation and substantially more roof insulation - but I live in the UK and my temperatures typically range from minus 10C in winter to plus 30C in summer.

You want Celotex type insulation, its rigid board with foil both sides.. I used 75mm stuff on the shed I pictured above. 4 (95mm) stud walls, which allows an air void to stop condensation forming in the cavity.

You need to go to a trade builders merchant. Dont bother with Bodge It & Quit (B&Q) or any other diy place.. The builders merchants will be a fraction of the price. If you want it to last and be sturdy use 3x2 (4x2 will be better) sawn treated timber for the walls, studs spaced at 400/500mm. Depending on the span, you want 5x2 minimum for roof. If flat roof forget the old roofing felt, its old hat.. Iv been using Fibreglass roofing system for the last 5yrs. Its a lot nicer finish, it has 25yr guarantee, it is slightly more expensive, and it is a bit messy, but its worth the cost and effort. Take a look on eBay and find the biggest bit of double glazed glass you can find for the cheapest price, use it as a roof light..
On the walls, use feather edge boards on the outside, and 9mm or 12mm ply on the inside. Fix the boards with 30mm screws every 6 to 8 and it will negate the requirement for diagonal bracing. You should really use a breathable membrane(modern day roofing felt for conventional pitched roofs) on the outside before you fix the boards.

The other option, which is cheaper, is to build it with concrete blocks. The only trouble is you may need planning permission depending on size. Timber structures come under different rules because they are not considered perminant.