Great answers so far, folks! Some expected, but some curve-balls too!
I ought to show my own hand, so here goes:
Believe it or not, I'm actually really happy with the gear I have. That doesn't mean I don't want more, of course
But I like what I have to work with, and it suits me. My only real concern is that, in time, what I'm happy with suddenly might not be available to me due to equipment failure. As such, some of my budget is dedicated to insurance...
Bodies:
Despite its age and obsolescence, my absolute favourite camera to shoot with is the humble 10 megapickle Samsung GX10 (same as the Pentax K10D). If I had to sell all of my cameras and keep just one, it would be this. I like its simplicity, and I love the quality of RAW files it produces. Resolution is, of course, very limited compared to modern cameras - but for me, it's not limiting, and I'll happily sacrifice resolution for the tonal response of that lovely CCD sensor. I already have two GX10s, both bought as mint low-shutter-count examples. One is still below average for its age, and the other remains as I bought it. I want to stockpile more so that they'll outlast me (!), but I backed myself into a corner with my terms & conditions - I can't buy Samsung branded cameras, right?
So, I'm going to eBay, Gumtree, and any other source available to find myself two Pentax K10D bodies with low shutter count. Typical asking price here in the UK is around GBP £150 (a little more than the Samsung version), so GBP £300 for two really nice, lightly-used copies.
Fast-forwarding to more recent technology (if not exactly state-of-the-art), I already have a K-3 and K-3II, and love them both. My K-3 has seen plenty of use, but my K-3II is barely "broken in". So, I'm going to buy another brand-new K-3II while they're still available. Coupled with my existing K-3II, that should last me many years. Current price for remaining UK stocks is just under GBP £800, which is almost 30% more than I paid for my existing copy... but my budget is time-limited, and I like the camera, so that's the way it has to be. GBP £800 - ker-ching!
Lenses:
Similar to my preferred bodies, my favourite lens is the Tamron 28-75 f/2.8. So long as I'm shooting APS-C K-mount, I really, really don't want to be without a good copy of that lens, so I'm going to buy a second one. They're becoming scarce to buy new, and prices are unreasonable, so I guess I'll have to find a decent used copy. I'm allowing GBP £250 for that, which is probably a bit more than necessary - but I only have a month to get one
I have the Tamron 150-600 in Sony A-mount that I use on full frame. I don't use it very often - certainly not as much as I should. But I have good intentions to change that, especially with the wildfowl parks that we have nearby. So I'd like a decent equivalent to use on my Pentax and compatible APS-C gear. The D FA150-450 is GBP £1,600 here in the UK right now. I'll have one of those, please!
I love my Sigma 30mm f/1.4 Art, which was inexpensive and produces much nicer images than I could have hoped for. The AF is also much better than expected, given some of the dreadful reviews. But I'd really like the Sigma 35mm f1.4 DG HSM, which I know is in a different class optically. It's currently GBP £650 in the UK. The price has dropped lower on occasion, but I only have a month to act, so I'll take it.
Accessories:
I have a decent Giottos tripod that I bought maybe seven or eight years ago. It's really good quality, but my, it's heavy. I want a good quality carbon-fibre tripod that I won't leave behind because of the weight. I'm budgeting GBP £250 for such a beast.
I'm up to GBP £3,850... and I'm going to stop there. That leaves me with £150 for minor price variations, or for a couple of silly accessories that may or may not be essential, but might be fun. And, per the intention of this thread, we must never forget the fun
EDIT: With the exception of my GX10 / K10D obsession, I reserve the right to change any and all choices at any time preceding receipt of the USD $5K...