Professional fit every time esp if you want a diesel hot system. Get it fitted into a brand new van if you plan on window cleaning for many years to come.
Nothing worse than messing about every few years switching systems from van to van.
I've had this van 16 years Daz but as I'm scaling down to doing less as I'm now only part time so for a start I don't need a 500 litre system a 350 will be fine I also want a nicer more up to date van etc which I'm hoping when I retire fully in around 3 years time I'll be able to sell for a decent price.
I would just keep your van another 3 years. My van is 20 years old, had it from new.
Unless there is something seriously wrong with your van it should last 3 more years of part time work