Would strongly recommend renting a van for a week or two, to see what things you like and don't like. Try to get one as close in size as the one you are planing on buying. If cost is an issue, see if you can get a cheap relocation. Will allow you to do a lot of km in a few days for cheap.
I have done two rentals (many years ago) of a transit van body based campervans like these:
https://www.happycampers.co.nz/campervans/happy-2-st/ Obviously just a 2 person campervan. Big comments were that the footprint isn't significantly larger than a ute, so while tight it could generally be parked without too much difficulty. Drivers seat back hit the bathroom, so couldn't quite get in the position that I (182cm tall) would like. Making up and taking down the bed is a chore, so we often just left it made up. Had a microwave and rear aircon that only worked when plugged in. Had a flued LPG fired heater which discharged hot air under the bed. Van insulation was really poor, so the heater only really took the edge off on 6 degree nights. Sadly the house battery (despite being a big 100Ah one) didn't last long enough to run the heaters fan & control system all night, so van would be frigid when we woke up. Drove fine, plenty of power to flow with traffic. Hot water was heated from either 240v or waste engine heat (not great for morning showers if you havn't been plugged in) - need to idle the engine for 20mins or go for a drive to get hot water.
More recently I have done a re-location in a flash 4 berth german built campervan for wilderness campers. Coachbuilt on a fiat van platform. Cira 6.5 - 7m long. Permanent island bed in the rear. Shower and toilet each in a seperate room. drivers and passengers seats on swivels to form a dining area around a table at the frount. Bench seat seat behind that table has seatbelts for pasengers 3 & 4 to ride in when underway. 2nd bed concealed in the ceiling and drops down over the lounge / kitchen with a touch of a button. Good insulation. Diesel / 230v ducted central heating / hot water system. Great insulation. Big cargo area under island bed and across the rear of the van. Just under 3500kg GVM so on a WOF and gets a 110km/h max speed limit. Enough power to flow with traffic on state highways. Downsides were: Big to park - we paid and displayed twice as we needed two carparks when we visited a wellington CBD restaurant. Big blind spots, too wide to stick you head out the window and look backwards - need to use mirrors & cameras or a spotter. Toilet seemed to need emptying every 2 days. Ergonomics of driving postion weren't ideal - wheel telescoped, but didn't tilt. Cabin door on right hand side, not kerb side. LPG fridge dosn't like being parked on a slope. No microwave or rear aircon (but everything works when not plugged in). Rear track much wider than frount track. Quite noisy on the open road.
They want to buy. you have not given them any useful advise toward answering there question