Welcome to gardening in SC!
1. No, you are not too late at all. There is still a chance of frost so the time is right for potatoes, onions, lettuce and probably broccoli and cauliflower. Wait a few more weeks for the beans and squash and peppers, unless you have floating row covers. Celery probably will not grow well here is SC - it prefers cloudy and cool, which we are not, but that doesn't mean don't try it.
2. All can go in the same raised bed if big enough.
3. Depends on how many plants of each. Raised beds seem to work better if they are long and no more than 4 ft wide (so you can reach into the middle easily).
4. I prefer untreated red cedar, but that's just me.
5. There is a bagged material at the big box stores for less than $3 a bag that looks like bark mulch but is called soil conditioner. This is great for tilling into clay soil, as is gypsum, another soil amendment. Compost is always good, no matter what the soil. If you're in a sandy area, peat and compost would be my choices. Just be sure to improve your soil now - it's so much easier than later, and get your own compost pile started ASAP.
I'm including the link for the Extension. You'll find lots of info there.
Happy Gardening!

Here is a link that might be useful: SC Cooperative Extension

You know what is both odd and funny?
You can begin a garden on land that has never been gardened(is that a verb?)with any crop and the darn specific bug for your veggie will show up.
Like they've been flying/crawling around for years waiting for you to arrive.
What do potato bugs eat while they are waiting for someone to plant potatoes???
Where do those white moths hide waiting for you to plant brocolli and cabbage??
Obviously, the growers that produce the starter plants you buy at nurseries and garden centers probably have sent plants with the moth eggs already attached.
Inspect your starter plants thoroughly, tops,undersides and in the crotches of the leaf attachment to the stem.
Be vigilant because the worm/caterpillar eats 'green' and is green and hard to see but overnight can decimate your young healthy plants.
Kinda makes one want to plant using seeds instead of starter plants to hide from the bugs.
They'll find you eventually.
ceezer..if you're on a hill you've obviously got some good drainage. I wouldn't go too high with the raised beds unless you are willing to be prisoner of the garden hose.
It will depend on whether you take some existing soil and incorporate it with bagged soil. Using only bagged soil will call for a lot of hand watering so you can forget about taking any extended vacations this summer.

Gardening is more about growing and maintaining great soil.
If you begin with bagged soil, whatever you plant will quickly use and exhaust all the nutrients. Adding compost that isn't completely 'cooked' will actually draw nutrients away from your plants. Soil conditioners make the dirt easier to work and help hold some moisture. Bagged composted cow manure helps feed the soil so I would add it in the same quantity as the soil conditioner.
Have the base dirt and then the mixture you make tested for pH and soil chemistry so you know what to add and how much to add.
Iron deficiencies,calcium deficiencies result in plant problems that will discourage you. If you know what soil conditions you have to begin with you can amend the soil and give your plants a better head start.
It can take years to build good, sustainable soils for growing crop plants.

Guess what, you have a great Master Gardeners club in Sumter. Contact your Clemson Extension Office there...they can help and also put you in touch with the MGs......Happy Gardening

Thanks for all the help, y'all!