The 1800kg max that you want - is that payload (i.e. boat + gear) or max GVW (i.e. boat + gear + trailer)?
If it's the former, you'll need to go for a larger than 1800kg axle, as you'll need to add the weight of your trailer to that 1800kg payload requirement. If you suspect the trailer to weigh 350kg then this would give you a GVW of 2150kg. You'll be hard pressed to get a single axle trailer to take 2150kg, never mind get a pair of single axle units / single beam axle to take that sort of weight.
As you've stated the trailer's currently unbraked, my suspicion is the trailer chassis itself is probably not up to the job of taking that sort of weight. You'll need to strengthen the chassis, winch post etc before thinking of upgrading the running gear.
To give you an idea of the sort of chassis you'll need, this is one of the largest capacity, single axle, trailers you can get. It's 1800kg GVW, giving a payload of 1300kg. You can now see that the chassis has to be pretty robust (as it weighs circa 500kg on it's own), it needs to resist flexing under towing and bending when you're launching and recovering.
If you're looking to buy the gear and do it yourself I can honestly see you spending £1k to get the trailer up to spec. Unfortunatley, the running gear is the expensive stuff - the rest of it is just galvanised steel!
I don't mean to pee all over your bonfire (no seasonal pun intended
), but I think you'd be better off spending your pennies on a trailer that's already built for purpose.
The Snipe listed on here looks pretty good for the money now, huh?...
- no vested interest,the way, just an opinion!
PS. 1563kg is going to be too much for most Mondeos and 1800kg is going to be waaaay too much!