You haven’t mentioned how old your boyfriend is, nor if he is fluent in written English (he isn’t doing the enquiring himself), this may have some bearing on the chance of success. It takes many years to build up experience in this game, being good with engines is only a small part of the job, more relevant is diagnostic and electrical ability and the old fashioned skills of repairing the bits that do fail, but which most garages just want to exchange.
Having re read what I and others had to say in both threads I have not altered my opinion that the time to set up as a mobile all makes HGV mechanic has passed, the opportunities lie in specialisation.
If you can find some genuine horse box owners who will spend money then that is a start, but the run of the mill Mrs Snooty will spend the earth on Dobbin, thinks her horsebox is worth a fortune because she paid a fortune for a wreck with a posh body on it, but wants you to find scrap yard parts for it. If only she washed it out properly the urine wouldn’t have seized everything in sight which has led to the argument over the amount of time taken. The ‘Tarmac yer drive guvs’ will run up a bill they won’t pay with every mug in town. This leaves you with the ODs and a quick read through this forum will give an indication of what rates they are working for or propose to work for, and hence how much money they have to spend; if they even have a vehicle that isn’t on R&M.
A read through one of the trade magazines like Professional Motor Mechanic, which are on the counter FOC at your local Motor Factor will give an insight into the gaps in the market. This magazine is aimed at the car and small van sector with articles from diagnostic specialists. The same problem with accurate diagnosis is broadly true for heavies, but since Texa and the like are being criticised the availability of dealer level diagnostics needs to be investigated. Even with that product knowledge is going to be vital, which means specialisation in one or perhaps two brands. To succeed with that requires detailed knowledge, which probably means two, three or more years in a main dealer workshop relevant to the fleets in your area, but keeping an eye on the age of the model range. Starting up S/E just as the new model range appears will be the wrong time.
If however there are two or three small own account 7.5t fleets operating the same brand locally then there is a chance of making a go of it. The insurance for a start will be much cheaper. This leaves trailers, which if one is never taken one on the road can enable a lower insurance rate can be negotiated.