Ciao!
I understand that is it complex, because some places want to offer you the entire villa or venue to actually give you privacy and not have other guests intrude, so if you look at a place that is too big you basically have to pay for all of the rooms. July is also high season, so the cost of the rental is highest in the summer.
First of all - as far as having a legal wedding here in Italy, have you already started looking into the documents you need? it depends on your nationalities (you are coming from Australia but I can't assume you're both Australians in today's global world). Sometimes getting the assistance of a local wedding planner even for that part is useful/necessary, given the language barrier but more than anything, also finding a local city hall to have the marriage in can get tricky if you don't have someone local helping on that end. Some villas actually are locations for city halls, so that means they can get the city hall official celebrate the wedding directly at the villa, but this is not a common practice so most places can only host the reception, not the wedding. I do know of a few that do, so I'll list those below. If you're planning on doing the paperwork yourselves, let me know if you have started already researching what you will need or need help with that. Many couples end up marrying in their own country right before the wedding, then having just an celebratory wedding vows done the day of the wedding party here, precisely to avoid all the paperwork (because you do then have to register your wedding back in your country once you return).
Now, as for locations: I have a few places to recommend that came into mind as I read your message so let me share them and why each one could work. These are smaller locations for the size of your party, or that have sections of the property that can be rented just for the wedding party rather than the entire property.
Agriturismo Incrociata - they have an estate that has both an agriturismo with apartments and a smaller villa up further up the hill from the main agriturismo. With a spacious garden around the villa and its own private pool, the Miravalle villa is ideal to rent for exclusive use for the wedding reception/party. The villa itself can host 6 people, so you could use the villa for the main wedding party family members and then rent a few apartments in the agriturismo for the rest of your guests and that way ensure everyone is close by for convenience. The apartments are literally just 5 minutes below the villa WALKING distance, so your guests would be able to get there and away without driving. The estate is located in Chianti surrounded by vineyards so a great area, but the closest "larger" towns for marrying would be Castelnuovo Berardenga and Arezzo. The estate produes wines so there are vineyards and olive trees around property. Of course, if you have enough other guests you could also consider the main agriturismo property instead if you wanted to rent all 7 apartments there for the wedding party.
Villa Medicea di Lilliano - this is a wine estate located just a few km outside of Florence and it actually is an official venue for city hall officials from nearby town of Antella for marriages. So the official wedding can be done there. The main villa is large, has huge gardens as well as a covered space so I don't think it is appropriate for your group (also considering the cost, since weddings are a big thing for them but larger size) BUT it also has a
separate and smaller villa further down the road that has a smaller garden amidst the aparments and its own pool. The garden is that one in the first photo in the linka above. There are 5 apartments there, and at that point you would need to rent them all to have the place to yourselves and not have other guests intruding on your wedding. Lovely view from the garden of the Florence duomo and its cupola!
Here, you could also decide to get married in Florence's town hall (Palazzo Vecchio) and not there at the villa, if you wanted.
Dianella - how many kids aside from the baby will be at the wedding? Because this estate is perfect for weddings but being a wine estate with suites for 2 people, it isn't really for families. It is also in Chianti wine country but near Vinci (famous for Leonardo). For a wedding, they would make you rent the entire villa - it is small and for up to 24 guests, so at that point you could have everyone stay there. I am not sure what the cost of renting villas of these types is so not sure it will be within your budget, but it doesn't hurt to write and ask. This one would most definitely not ask for week stay, since it functions as a hotel. So you can rent it just for the 3-4 nights you need it for. I've actually been at a wedding there and it is magical, so also consider that these estates will also often organize the location for you: all the decorations, the table settings, the entire menu and cake, flowers, etc as part of the service of renting the villa when you book directly with them. If you don't, you will have to look into doing all of this organizing on your own or hiring a wedding planner to do it for you. Dianella's owner actually worked as a wedding planner many years ago so Veronica definitely knows how to make a wedding special. I also think you could have the official wedding there and if not, then at Vinci which is very close by.
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Piaggia is another estate owned by the same family, it is a villa rented as a whole for up to 20 people so it is another possibility. This one is located in Chianti headed towards San Gimignano. This one for sure would give you great location and privacy, but I do think they rent is by the week. You should still write and ask about both, considering they might not be available for your dates so you have to contact as many as possible.
Ok these are the ones that immediately came to mind so wanted to send those off, have you contacted any of them? I'll keep thinking of any other smaller properties where a wedding would be perfect for your needs, so let me also know any other questions or doubts you have that I might help with.
