We're planning on visiting Charleston, SC at the end of May. I was just looking at King Charles hotel, oh my gosh, it's over $250 per night. Can anyone recommend somewhere clean, even if it's in the outskirts of Charleston, we will have a car. I would really like something under $150. It's for (2) seniors.

You'll be visiting during the extremely popular Spoleto Festival USA which runs May 27-June 12, 2016. https://spoletousa.org/

Because of this, lodging is higher priced and fills up quickly. If you can't find anything in the historic district (which we all recommend) then try any of the Lockwood Blvd. hotels on the west side of the peninsula, or Mt. Pleasant, Daniel Island or West Ashley areas.

Holiday Inn Express/Ashley includes breakfast and is about a 20 min walk, but there is a trolley nearby. They should have a senior rate in your price range.

Cheaper still is Clarion Mt. Pleasant which I think includes breakfast and there is a mini-fridge and microwave in each room. The location is fine but you'd have to drive into Charleston or take the $10pp water taxi or a cab. I think I would book this one while the prices are low, you can cancel later if you find something else.

Just a question--did you mean to come during Spoleto? It is an absolutely wonderful event, but if you aren't specifically interested in it, maybe come a different time, if you can arrange that.
I would choose Mt. Pleasant if not able to be in Charleston proper.

We stayed here. They have a senior rate, but you have to call for it; not shown on website.

Great breakfast included along with a happy hour with appetizers.

Clean, great location, good sized rooms. We paid around $150 in early September.

http://www.indigoinn.com/

We'd rather go when there is no festival going on but we might be tied into those dates. Is 2 days enough to visit the sites?

What sites do you want to see?

Be aware that Charleston has a lot of traffic and getting from point A to point B generally takes much longer than it looks on mapquest or the like. I find it exhausting to get around.

Well, 2 days will be "ok", if that is what you have. There is much to do, see, and especially enjoy food wise. I'd choose one plantation--we like Middleton. It will be super crowded everywhere if it is Spoleto and will take even more time getting from a to b as suggested above.
Plan what you are going to see in advance.
If you want a special dinner decide and make reservations well in advance.

Would Mt. Pleasant be a better area to stay in? The hotels are less expensive.

better than what? Three posts above recommend Mt. Pleasant.

If you can't stay in the historic district, then Mt Pleasant hotels and the hotels on Lockwood would be the ones I would choose first. Then those hotels in West Ashley and last the one on Daniel Island.

I feel that each area has advantages. Mt Pleasant is close, but busy with traffic. Lockwood hotels have the advantage of being on one of the free trolley routes into the historic district. West Ashley places you nearest to the historic plantations should you wish to visit one or two of them and you'll avoid in town traffic getting to them. Then the one hotel on Daniel Island places you with easy access to the 526 loop, then to the airport should you be flying in /out that way.

You said you'll have a car, so maybe Mt Pleasant, then drive into town and park at the visitors' center, take the free trolley around and you won't feel the stress of all the traffic, which is incredibly bad and slows down the day's schedule. It will be worse during Spoleto, so arrive early into town and park your car.

As far as two days being enough--you'll wish you had more time! Charleston has a lot to offer. But if you organize those main sights that you really want to see, and do them first, then you might get a feel for the wonderful historic nature of the place. I think that with such limited time I would see only the sights in town and not go to a plantation. That is of course, unless you really want to see one of them. You'll have to decide what is most important to you.