One thing about all the HO kits, they provide many ideas that you can extrapolate to
O, and some can even be picked up cheap enough, to get the plans, that you can
take to Staples and blow up for a scratch build of a model for which no O scale kit
exists. I have a model of the famous mine at Red Mountain, Colo. under construction, with its HO kit handy. (but some HO kits are rare, too)
I agree. some of my best builds were using HO scale designs. I find it very easy to "scale them up" to O scale using only the pictures. depth and height are a reasonable judgement call based on whether it's a commercial or residential building. your eye will tell you if it's right (e.g. you wouldn't make a single story residence larger than a factory). Floor height can be approximately 3 inches. Using O scale windows and doors from Tichy or Grandtline will make everything look appropriate.
for my tastes, I like wood builds (kits or scratch) as the material is easy to cut, is very forgiving if you make a mistake, and uses only wood or tacky glue (no glueing your fingers together with ACC). it's also highly realistic and these days you can even get brick texture walls in wood. I also love downtown deco kits as the castings are first-rate and the hydrocal takes weathering just about better than anything. most of their kits are very easy to assemble and the trick is painting and weathering but their instructions on these techniques are extremely good and detailed. like with any kit, read all the instructions first, take your time and enjoy it!
not that most of us build for anyone but ourselves, but there is definitely an appreciation factor from guests when they learn you've built the structures and created these interesting scenes, something you might not otherwise get from laying track or electrical work.
Jerrman