You haven't said which hotel you're staying at but whether you stay on-site at a Disney hotel or off-site, you'll have to pay parking fees. I think it's $25 per day at the park - pay once, it covers you at any Disney park that day. If you go to the park every day that will be a sizable chunk of your £500 extras budget.
Look at Discount Florida Car Hire for your car rental. Their website prices are high like everyone else's atm, but if you email them for a quote they tend to give you a much lower price. I saved £300 doing this for my June car hire this year. They have a very good reputation on The Dibb website, a British Disney forum which is an absolute god send for planning your first Disney holiday, esp if DIY.
Also use cashback sites TopCashBack and Quidco. I will be getting money back on my park tickets (bought at AttractionTickets.com for cheaper than Disney sell them), car hire and villa hire (we're doing 5 days onsite, 8 nights in villa).
Your price could be reasonable but it's hard to know when you haven't said which hotel it includes, where you're flying from and whether indirect or not. It might be worth getting a quote from a really experienced Disney travel agent like ForeverAfter (I think that's their name - Google MyMagicHolidays as that used to be them) just for comparison purposes. We booked quite recently and are paying £9k for onsite at Caribbean Beach Resort for 5 nights, v nice villa for 8 and car hire for days. Including WDW park tickets and genie plus with direct flights. Before the pandemic we'd booked to stay for 14 nights at CBR in May 2020, no car hire, direct flights and that included the Disney Dining plan (all our meals basically) for total of £6,500 - so s lot cheaper than the rebooked and tweaked holiday is costing. I would say that prices for everything have really shot up. You should download the My Disney Experience app and you can view onsite park food prices on there. Extortionate doesn't begin to cover it, and tbh that was before the pandemic. I think your spending money expectations are optimistic but I guess it depends just how strict you can be with yourselves.
Happy planning - it's expensive but hopefully worth it. Crowd levels are currently v busy, to the point I'm not sure I'll be disappointed if Covid stops us from going and we have to defer a year!