There is no reason why you can’t keep To Do lists in Scrivener, if all you want is a collection of flat lists of text items – in fact, I used to use Scrivener for this myself. The binder gives a nice, hierarchical structure for organising your lists (for example, into the categories you mention), and I simply deleted items as I completed them. Obviously, you don’t get the task management functionality of a dedicated program like The Hit List (my current favourite of this ilk), but you can certainly use Scrivener in this way if you just want a basic implementation. I imagine it would also work for the Autofocus task management method which someone mentioned here on this forum a while back.
In your case, you could create a folder called “To Do”, containing separate documents for each of your various categories (outside, office, shed, garden, vineyard). Another folder could be “To Get”, containing either a single document listing all the items you want to buy/acquire, or a number of documents separating your list according to (for example) timescale or source. The ability to add document notes, related URLs and so on might also be useful here, and if you create your folders in the Research area, you can add PDFs or web archives which might be extendable as a sort of inventory of receipts/invoices, or a catalogue of brochures related to your “To Get” list.
It’s not especially unorthodox, but I have a Scrivener project full of things that I might (or might not) develop into proper writing one day. Basically, I use Scrivener to hold all those bits and pieces of random text or “notes to self” in one place so that I can see what I have got and impose some sort of structure on it. Split screen is useful because I can view or work on two documents at the same time.
I have never used Scrivener for general correspondence, but I don’t know why not because the binder would be a useful organisational tool (allowing grouping into folders related to particular projects, for example), and Scrivener has all the usual formatting things for general text work. You can print straight from Scrivener, and can use Print Preview to check that everything looks reasonable. Once you had set up a document with all the formatting you want in your letters, you could simply duplicate it each time you wanted to write a new one, renaming and moving the new document as appropriate.
To some extent, Scrivener can be used as a sort of document management framework and a receptacle for just about any sort of document. However, I would be very reluctant to suggest that it is exactly what you are looking for, since you may not think so when you use it for your own purposes and since you would be using a tiny fraction of its functionality. Your best bet is to download Scrivener for a free trial. You get 30 days of actual use, which is easily long enough to know if you can get it to do what you want – and if you don’t like it, the files you have created will all be in RTF, so you won’t lose access to them.