Regarding Herms...it's extra work to build, for sure, a circuit, another manifold, the heat exchanger itself, routing through the pump....but..once it's done no more extra work each brewday recirculating wort by hand in a jug that sloshes, and can be dropped, and watching the mash cool down as I do it. No doubt I'll try out my idea and also a proper HERMS mash ending up doing that which suits me best.
The truth is I'm lazy and if I expend a lot of one-off effort building it - not to mention the enjoyment of doing so - then I can be a lazy git on brew days as most of the manual lifting, carrying, jugging, and pipe connecting work would have been removed leaving one free to concentrate on the important aspects of brew day, namely eating fresh bread, cheese, and drinking beer of course...oh, and the actual brewing process.
Get a builder in? Good idea! Good job I already have one in the form of my father in law, a builder/carpenter, recently retired, who is kicking his heels and itching to be doing something! I built our house extension largely with just Bill and myself and over the years I've aquired skills in plumbing, bricklaying, electrics, carpentry, tiling, and decorating from him.
This brewery is a big project, for sure, but it's very do-able. It'll just take time...and money. Support from the Memsahib is obtained by continuing to make a brew-mess of the kitchen at dinner time, over and over again

Then she can see the sense of me moving to the shed!
Initial work won't have much to do with brewing per se. It'll be about making the shed a fit place to do it. Brewery comes later. I'll post pics as I go.