So exciting. I hope the situation works out in a positive and beneficial way for you and the horse.

A lot of good advice here so far.

The rough condition of the horse, I'm not sure what you mean, but if the horse is in good condition and has been wormed regularly and has gotten any necessary vaccinations for the area, it's very likely he's just rubbing his neck and mane on trees to get a nice itchy spot and that makes the mane sometimes get sticky and twirl up into zillions of 'granny knots of the century'. You can work some of the bigger knots out with your fingers, then use wide tooth comb and then comb water through it to get it to lay smooth. This is a project that can sometimes take days and you need to realize you have all the time in the world. Be patient and careful.

Don't know about the bits on the legs that you mention as that seems a bit vague. But horses have these crusty scab like things called 'chestnuts' on the legs. They are natural if they look similar to this:

article that goes with that is here:
http://www.ehow.com/how_2091482_trea...chestnuts.html
I would advise that you not worry about them if they are chestnuts. You could get hurt trying to do what the instructions say (your degree of experience foremost in my mind) as it is really just a cosmetic thing in most cases.

HOWEVER, his feet are an area that may need attention. The owner can probably advise on that and may be able to teach you how to halter the horse without getting hurt, how to lift the feet, pick them out and to check for hoof condition, and also show you how to give a tube of wormer without breaking your neck if the horse fights it. If the footing is naturally rocky, and the horse has good conformation and good hard feet, then the feet will tend to wear down normally and not need extra attention. If the ground is soft however, you may need to find out about having the feet trimmed a few times a year which incidently are also times when he can also be wormed.

It is some work to have a horse there. Realize that a horse accident can put you into the hospital so you should not try to do anything without having good guidance. (eg, do not go barefoot or in open toed sandals around horses, as your toes might get crunched). Don't stand too close to the rear of a horse as you might be close enough to be kicked accidently.

Learn to read the horse's ears. If they are 'watching you' then so is the horse. Watch where the ears go and how relaxed or tense they are. The eyes and mouth can also show tension or relaxation. Horses have nice itchy spots on the neck and withers that you can scritch with your fingers. And they usually blow a soft greeting into your breath if your body posture invites it. (dirty fingernails are a way of life for me around horses, )

Manure pick up and other things too, may need to be considered. Flies clan make life with a horse property take another dimension.

Just for kicks, here's a pic of our boarder's minihorse and her new foal.