ok lots of general questions to which there are lots of general answers, depends on what you intend to do race or bash?, 1/8 cars are particularly hard on servo's ,the losi 8 is really hard on them if you don't mount them properly
hitec servo's probably use a 645mg around £30-35, i use KO 2343 around the £100, put probably the best price to performance servo would be the ds1015 around £60-65.
glow plugs and fuel depend on the engine, nitro content from 16 to 30 percent again some engines like less nitro some like more.
pretty much all buggy wheeels come with a standard hex and offset so any wheel willl fit any car, same with tyres, although again what surface you run on will dictate what tyres you would want.
depending where you live go to your local model shop and ask their advice