I have a 3.5 van conversion - it's really economical on fuel - better than my car when it's unladen! I have insurance with Shearwater along with the top level recovery policy which is supplied by Equine Roadside and Rescue - I think altogether it's about £350, but if you go without recovery I think it was about £180 just for the insurance, although I had to claim for a bump in my car the year before, and they told me it would have been cheaper if it hadn't been for this!
My tax is £135 for the year - cheaper than my car.
The most expensive part for me has been the repairs. In two years I have had a new clutch, new gearbox bearings, new alternator and new cambelt - total in the region of £2200. Hopefully that's the main things done and for the past 12 months it has been fantastic.
3.5t are great for just one horse, but you will be overweight with 2 in. And the lifespan won't be as long as a big tough lorry, but there are lots of pros and cons in both - you just have to weigh up which is better for you....