Depends where you are in the country as to what grass livery makes.
I've never paid more than £10 per head per week for grazing, and some farmers charged me £4 a head for grass or £2 for stubbles as that is the rate they charged for cattle of similar size/intake/ground damage. When renting acreage rather than per head the going rate is around £90-100/acre for the season but I had 20 acres of hill pasture for £110-150 pcm (over a 9 year period) including running water, year round. The landowner responsibility for maintaining stockproof fencing and promised shelter/storage never materialised and I supplied my own fodder over winter, but
If you're not experienced in keeping horses then I wouldn't impose too many rules in case the owners actually do know more. Some are numpties sure, others will try it on, but knowledgeable horse owners will be put off by rules that make no sense or are not open to negotiation if they have a good reason for a request in terms of welfare etc.
Many horses will respect a single line of electric fence, but some won't, usually youngsters are the least likely to stay where put. If your wall is a stonewall and securely built, it would be better wind and weather protection without a tape, if it isn't secure then a stock fence would be better. Trees may get rubbed on or nibbled but most grown ones will be safe enough other than the likes of willow which is medicinal. Any conifers should be completely fenced off as poisonous to horses - particularly leylandii, and if you have oaks then they should be fenced off when acorns are present as those too are toxic to horses. Mature sycamores, ash, beech etc I've never had damage to.