Personally, I love carts and have one myself. They filter almost as well as DE. As far as maintenance goes, it really depends on whether the filter is sized right. Most carts have historically been undersized for the the pools they are on and needed frequent cleanings, sometimes as often as weekly! If the cart is sized correctly it should only need to be cleaned every 3 to 6 months (or possibly only once a season if the pool is not open year 'round.). FWIW, I can run my cart for a year without cleaning and still not have more than about a 3 psi increase in pressure (cleaning is required when pressure rises about 8-10 psi or at least once a year, whichever comes first). FWIW, a "dirty" cart actually filters better than a clean one so you don't want to clean it TOO often.
Soaking is really just once or maybe twice a year, depending on the organic load of your pool. The trick is to have two carts (or set of carts for a multi cart filter) and to swap them out at each cleaning. With proper care the carts will last 5 years or more. To clean then get a 3 dollar short, small bore brass garden hose nozzle. Works better than the expensive cleaning wands and gadgets. As far as sizing goes, what PoolDoc said for sand also can be applied to carts. Personally, I like to look at the commercial sizing instead of the residential sizing but that is probably overkill.

As far as parts for your Purex DE, Pentair owns the Purex name and many of the filters that were manufactured under the Purex name are still in production under the Pentair name. DE is still going to be the best water filtration you can get. FWIW, breaking down a DE filter is more work than cleaning a cart in my opinion.