I do 14 litre batches, which give me an estimated 11 litres into bottles (always hit the estimate in the past too), which is 22 500mls bottles. I start with a normal recipe, put it into Beersmith 3 (which I chose as it had an app and desktop software, and with the sub you can upload to their cloud server, then access the recipe on both desktop and phone, and use phone for timers etc, just personal preference really, although it does annoy me that the only acid additions it supports are lactic, phosphoric or acid malt...) as if I am going to brew the full volume, then scale it to my equipment profile and the batch size I want, et voila.
I started doing the smaller batches last go round, as found that 23 litres was giving me more beer than my wife and I could get through, and if it was one we weren't keen on it was even worse! So little and often was way better for us, and allowed more variety.
As to boiler, my advice, do yourself a huge favour and save up and buy a single vessel system if you aren't going for multi vessel. I did the whole process of boiler - DIY build eBIAB rig etc, ended up costing more and was way more work too. Bought a system just the other week finally ready to get brewing again.