I've made two conversions to Rohloff-hubbed bikes, one to a front wheel drive electric motor, one to central drive electric motor. The front wheel motor conversion is described in detail starting here:
http://coolmainpress.com/BICYCLINGbuildingpedelec1.html Something similar would be an easy conversion on a Raven or other EBB bike with whatever transmission including Rohloff, a couple of hours max. My Bafang/8FUN kit was totally complete, including the battery in an oversized water bottle format, and bought from a British dealer for STG 425 IIRC.
To fit a central motor on a bike with an eccentric bottom bracket, all sorts of contortions are offered. I wouldn't touch them; bicycle engineering should be simple and user-fixable.
To fit a central motor which hangs in the bottom bracket shell, in my opinion the best solution, you must solve the problem of the chain tension adjustment, probably bringing back the sprung drop-arm and jockey wheel, ugh. But the central motors in the aftermarket are better than the motors available for the front wheel, and a better cycling solution too.
I seem to recollect that we recently heard from a forum member who ordered a conversion kit from SJS -- you should look into it. It may be a bit more expensive but you know it will fit and it will work, and there's someone to help you at the end of a phone line.
That said, it shouldn't take even an amateur with a bare-bones tool kit more than an hour or two to fit an electric motor and its complete kit of controllers and the battery. The key word is "complete".
The main difficulty, once you have a good kit and have solved the chain tensioning problem on a Raven, is -- hold on to your hat -- making neat wiring for the many control cables. I considered shortening so many cables -- and bundled up the excess out of sight inside office-type helical plastic cable collectors, which look like they're part of the bike.
Without commenting on the relative prices* I just wonder if converting a bike that will soon be 20 years old makes sense. There's the attraction of the familiar and comfortable, of course, but you'll be introducing new stresses for which the bike wasn't designed to parts that may have been stressed already.
Good luck and don't hesitate to ask about specifics if you decide to make the conversion yourself.
* It rubs all the right people wrong; see
https://groups.google.com/g/rec.bicycles.tech/c/LqY0UpZTIP0/m/I18KpJSsCQAJ