I can't help with insurance costs, our business is covered under the Farm Policy. Don't forget Property Rates too.
Otherwise, maintenance, materials, machinery (buying tractor/quad/trailers) wear and repair on them, fuel, harrow, roller, mower, hay rake, stocking up your workshop with all the DIY bits and pieces you will need etc
- just when you think you have everything ticketty boo, all the fences up etc, some pratt of a horse will stick its' stupid feet on a gate and trash it, or the wind will blow and a tree will fall on your fence, or barn, or some twerp (talking from bitter experience here) will crash their car through a brand new fence.
It is never ending, whatever you budget for, add another 50% on that. Better than going out to work though
It's a lot more than you'd expect. You need public liability insurance, and depending on what you do in terms of livery, may need care custody and control insurance. Then you have business rates, electricity, water, maintenance costs. Fencing, stabling, arena, lighting. Equipment such as forks, shovels, wheelbarrows, horse transport (as if you have full liveries you would need to provide transport should there be an emergency). Feed, hay/haylage, bedding. Costs of your own equipment such as work clothes, boots and gloves (you get through a surprising amount). Laundry, tack and training equipment. Tractor/quad, areana leveller, paddock clearing equipment. Fuel. I'm sure there's more. Tax, NI and staff costs (if a large yard).