More amateur advice. I don't think you have enough cash. If you really know piano internals, you can find a nice grand for 4k that you can keep in shape yourself relatively easily and cost effectively. For now I would DIY work on the upright you have to get it playing as well as you can yourself. Build up your expertise. Once you know how difficult it is to make a worn out action feel the way you are happy with, you will hopefully rule out lots of lost causes. There will be a 60 year old grand languishing somewhere, hardly played, in your budget. I saw a nice Schiedmayer grand, turn of the century, in your price range at a local sale. Owned by a local University Professor and part of an estate sale, was maintained and played OK, it went for 3K, but only because I was bidding also. Might have gone for just over 2k without me there. I let it go, as I was looking for something almost for free.
I see Steinway Grands needing restoration advertised in the US for $6K to $9k. I have played quite a bit on a little used Hamburg Steinway almost 60 years old. The touch is sublime. The stuff in my local dealer, rebuilt Steinways, Bosendorfers, Yamahas, that have seen many concerts, don't have that 'new car' feel that this old Steinway still has. My approach would be to look at a few of these restoration projects, and find one good enough to last another 20 years or so on a DIY basis. It doesn't have to play as good as new, or hold tune for more than 2 months, just play better than some tarted up 35 year old import for sale at a local dealer for the same money. If you picked one with a nice action, one that has not been restored, and can tune it yourself, you could be on to a winner. Ignoring sound quality which is subjective, in the 60 to 90 year old grand piano market, Steinway were one of the best made, which makes it easier for you to work on, and hopefully more reliable.
I would be interested to hear what others think about the above, having never actually tried it myself.