they will keep your lawn mowed and eat pretty much any other plants as well.
There will be sheep poo everywhere which will attract a lot of flies
They will rub themselves on any structures leaving wisps of wool everywhere.
They will need to be sheared and you will probably have to learn to do it yourself of bribe a local farmer to have yours done at the same time theirs are done.
If you keep them in a small area and don't have room to move them to fresh pastures frequently they will need supplementary feeding in the form of hay/ haylage/ silage and possibly concentrates and licks. They will probably need this in winter regardless of how much space you have.
They will need treating regularly for clostridial diseases. This will mostly involve you or someone else having to squirt drench down their throats which they don't like and definitely won't stand still and patiently while you do it so you will need to have some sort of way of penning them while you do it.
Ditto treating them for mites/lice/fly strike etc. This involves either dipping them in lethal stuff, injecting with lethal stuff or pouring pretty lethal stuff on to them - see above regarding needing sheep handling equipment.
They will get bad feet and you will need to treat this. This often involves dealing with maggots. Ditto fly strike.
You will need get a CPH number from the rural payments agency and you have to register a movement every time you move them off your CPH.
If they die you will have to have them removed by a fallen stock operator.
depending on the breed sheep wool has mostly little value but has great uses in gardening such as in mulches, frost protection and it can be composted. You won't have a garden left to do any of that if you have sheep in it but you could always pass it on to a neighbour.