I've owned a Grampian 26 for many years, and and O'Day 20, Precision 23, Catalina 28, etc., and am currently the owner of a Catalina 315. Sailing Lake Michigan out of Muskegon.
Docking will be a little different. You'll probably need to do some practicing and learn to cope with and use the prop walk.
You should be able to single hand it if you were able to single hand your O'Day. All the parts will be bigger...bigger Genoa, main, winches, and so on. Flaking the sail will be more of a challenge. Lazy jacks will be helpful.
One of the biggest differences will be going from an outboard to an inboard. Something goes wrong with an outboard, you can unclamp it, toss it in the trunk of your car, and take It to the mechanic of your choosing. Not so with an inboard. You'll be subject to the expertise of the marine service provider in your immediate locale. So, if you don't have some mechanical knowledge and some handiness, you'll need to develop it. You should certainly be able to change your oil and filter, change your transmission oil, do your own winterizing, and be to change fuel filters, and be able to replace a raw water impeller. And, when you have some sort of problem, it will benefit you greatly. if you can, with the assistance of the internet, effect a cure. At least for the simple stuff. You can have your local service provider do all this stuff if your checkbook will withstand it. An outboard is a reasonably good solution on the Great Lakes for boats up to 27 feet, and they have inherent advantages and disadvantages. I think the advantages tend to be overlooked.
If you can handle a 25, you can handle a 30. Unless you're a total putz. Then again, I see a lot of total putzes on Catalina 30's around here.
Bear in mind, when you take your 30 out for the first few times, everything doesn't happen at once. Motor it. Unfurl the Genoa. Sail it that way. Raise the main on another day. Ease into it.
Your expenses will go up. Longer boat equals more storage fees. More bottom paint. More engine expense. Sail replacement costs more. Running rigging is more. Etc.. But, they won't go up a lot.
I've been very happy on the Great Lakes with boats ranging from 28 to 32 feet. Big enough to handle the lake, and small enough to handle relatively easily, and when you sail to another port for the weekend, you're not trying to dock a behemoth at a strange dock.
All this assumes you get a good Catalina 30. There's a lot that can be wrong with any sailboat. Pardon me if you already know this, but a thorough buy pre inspection is important, as is getting the best available survey performed on it (you'll probably need it to get insurance, anyway). If memory serves, in 1986 that Catalina quit using a plywood spacer between the hull and the bolt on keel, The plywood is likely to eventually become waterlogged, and the "Catalina smile" will become evident.
They've made so many of these boats, you have the benefit of a large user group to get you over the rough spots.
PM me if you want to visit Muskegon and take a sail on a 315, if you haven't already tried a boat of this size on the big lake.