I think you'll be fine, you said you have reasonable DIY skillz and sumps aren't that tough to build on your own. When I set up my first SW tank (90 gal, FOWLR), I built my own sump for it and it's been working great for me ever since.
Just be sure to do your research, it sounds like you have been, and consider the design options you have and figure out what will work best for you. Once you build it, it will be a pain to change if you wish you had done something different.
Most sumps have three chambers, the inlet/skimmer chamber, the refugium chamber, and the return pump chamber. I personally just place the heater in the skimmer chamber (you could put it in the return pump chamber just the same), so no extra chamber is needed just for the heater. The order in which you place those chambers is up to you, the most common arrangements are either skimmer->refugium->return pump, or refugium->return pump<-skimmer (with the overflow from the display being split between the skimmer and refugium chambers).
Give yourself as much room for the refugium as you can, you want at least 10% of your display volume, and the more the better. The skimmer chamber will need to be large enough to fit your skimmer in (of course). And the return pump chamber will only need to be large enough to fit the pump in, but there are a couple of other things to consider in the return pump chamber: mainly, the return pump chamber is the only section of the entire system in which the water level varies as water evaporates... so be careful that it holds enough water to allow a days worth of evaporation to occur without exposing the pump (if using an under-water/non-external pump). Unless of course you plan to use an auto-topoff system of some sort.
Good luck! I had a blast researching and building my own sump, it was fairly easy and I made a pretty good one with no experience (other than decent DIY skills like you