As far as I'm aware welded steel section and precision rail don't go hand in hand , you will need to grind the steel or shim the rails to ensure that the precision you have bought is actually achieved. I'm not saying that welded section is bad just that you need to be aware of the work required to get the best out of your rails. (bolting precision rails to bent steel = bent rails)

Cutting forces in hard wood can be high, what wood and depth of cut are you looking at?

If its mainly a router that you are building then I would make sure that the dust extraction is very good, one of the main problems is re-cutting of chips which can result in poor finish and short tool life. for example I have just cut alot of edges of a kitchen work top with cheap cutters, after each pass the dust was removed and the cutter had a clear pass. one evening in a rush I didn't bother with hoovering after each pass and the cutter was useless after one joint.......

Good luck with your build and the best advice I can give is to buy the best you can afford in the first place as you will only end up paying out double.