If you need to get a ride on lawnmower on the back, you might need a pickup commercial or a high roof van, or smaller box back van, best thing to do is take the overall dimensions of the mower, get a couple of brochures with internal dimension and compare, or go to a few dealers and check them out in the flesh.
There are loads on the market, if you decide on a Double Cab pickup be ware that you need to pay 13.5% vrt (which will be included in the price)
Compared to a van which is 50 euro flat rate. I suppose it depends whether or not you want to carry more than 1 passenger.
Because you already have a nissan, then maybe look at nissan commerical models first for trade-in, but it probably won't matter too much in the end.
also diesel engine is best for pulling trailers