We've been about 14 times now because we love it, so maybe a little bit one sided on my part but I will try to help a bit.
£30k can easily be cheaper. We always book our hotel & tickets directly through Disney but then buy our flights separately (always fly Virgin Atlantic, but you can get cheaper again).
You could also look for hotels/resorts off Disney property which do come in a lot cheaper. Some hotels near Disney Springs (the shopping and restaurant area) have partnership deals with Disney, so you do get some of the extra perks that the Disney hotel guests get.
On your first day, get to a Walmart and buy a load of snacks & bottled drinks. Cheaper to do it this way than buying them in the parks. Although if you're just drinking water, you can get free cups of ice water at most food counters in the parks.
100% worth it for us as a family. We've been loads but still find lots of new things to do. We venture outside the parks too, so we'll do Disney & Universal, but we have also stayed in a Radisson hotel near Port Canaveral for a few days, hired a car & had beach days, visited Kennedy Space Center, shopping trips etc.
We've also done a few Disney cruises from Port Canaveral to their island in the Bahamas. The cruises are amazing, but an added expense.
Disney have 4 parks plus 2 water parks. Get the best ticket you can afford, so park hopper, 14 day ticket.
Universal have 2 parks & 1 water park.
If you stay on Disney property they put on buses, boats, monorail or skyliner depending which hotel you're in & never takes a long time to get to parks. The furthest away from most parks was the Animal Kingdom Lodge, because they have animals on that property the hotel is further out. Our favourite hotel is Yacht & Beach Club, but Port Orleans French Quarter is also lovely. Saratoga Springs is great location etc but very difficult to get a room now because they release a lot of their rooms for the Disney Vacation Club now, which is like a Disney timeshare scheme.
Universal takes a bit longer to get to, but off the top of my head I think it is around a 45 minute car ride. We always book an Uber for this, but I know you can get buses from certain locations.
Don't go to Seaworld. If you do want to go to a sealife center, do Discovery Cove. It's beautiful and they have limited numbers every day, so doesn't get busy. It is expensive though on top of all the other expenses.
Age wise is a difficult one for me to answer because we have been taking our children since they were babies. Our oldest was 6 the first time we went, but our youngest was not even 1. I will say though that we all love it there and we all love to go back as often as possible. They both love most of the rides so that makes it easier. When they're smaller it can be a pain not being able to go on certain rides, but there's so much to do in each park they would soon get over the disappointment of missing out on one or two rides.
Car hire is not essential. The only time we have ever hired a car is when we stayed in Port Canaveral for a few days, but even then we could have easily just used Uber. We have never hired a car while staying on Disney property.
I could go on and on! Please ask any questions as you think of them & I'll come back when I think of other tips/tricks :)