Body type will not have effect on taxation class, it will be PHGV tax £165 . as prev post , it does look like a bus/coach . They want to move it to HGV as a tax class as you cant have private coach/bus tax , and it would not be possible to class 4 mot it , as far as I can work it out ? They have confirmed it complies as a motorcaravan so you quite safe with class 4 mot .
I currently have a 5.5tonne Iveco welfare bus conversion, which the DVLA are happy for me to use as a motor caravan, it has a class 4 mot, & its taxed as a bus. The post office couldn't tax it as anything else while that logbook taxation class says Minibus. The entire system is shit, everyone interprets the rules differently & nobody, including the DVLA seems to have a clue what the actual rules are. I get different advice every time I ask anyone, its just a minefield!
Just do the conversion, present it to a HGV testing station, who will test it as class4, they have to test it as presented.
Tell your insurance company its a converted bus, they don't care any more & may ask you to try to get your logbook changed within 90 days, or they may not. Tax it as a bus its £165 a year, or PHGV,which is the same rate. Then fill it with diesel & get out on the road & enjoy.....