That's a large amount of money at stake here.
On approaching bill splitting for the venue
You say you would not mind paying the whole amount if you knew it'll would come back to you should you be in need of something (whether it is an act of service or money, I guess from your question). The thing is, unfortunately, you can't know for sure whether it will, in the end.
I suggest you send them a mail. This mail should include:
- The information that you found a venue for your next vacation all together. It implicitly shows your implication in the vacation organization on your own.
- The price all in all and per person. If you only give the full amount, it may be difficult for them to immediately know how much they would spend in housing for that vacation.
- A link to the property you found, so that they will be able to check it on their own and see whether they think it would be a great place to spend their vacation with you all. Be careful though, the more friends you are, the more people will likely give objections to the property you found. To avoid this, I suggest you add something along the lines of:
Should you have any issue with this venue, please let me know, and together we'll find something that's best for all of us.
This explicitely says that you don't mind to look for something else, yet it asks for their help to find another property. That way, you won't have to deal with all the organization of your own. This is something that may be difficult to avoid when you plan an event with other people: if they see you're already organizing the event, they'll likely think that someone's already taking care of the organization and they don't have to get involved in the organization. If you choose write something like suggested above in your mail, it should avoid people to think that you'd like to deal with all the organization on your own.
On assuring them you can adjust the amount paid according to everyone's abilities
You say you'd not mind paying more than your share should it help people who may not be able to pay all of theirs. I think that the third point of the email already indicates that you're willing to find something that suits all of you, whether it is about the accomodation in itself or its price. Yet if you want to insist on it and clearly say you're willing to pay more if needed, you may want to add something along the lines of
If the price is an issue for you, please send me an email as soon as possible and we'll try to find a solution.
There's no need to be verbose in your message about what you're willing to do in order to help them, as it may embarrass them and they may think they'd appear as poor, or stingy, in front of all your friends. If someone indeed sends you an email telling you they have issues spending that much money in a venue, I suggest you tell them that you can find another venue together, or else you're willing to pay for the part of the share they can't afford. This way, it indicates that the most important thing is that you get to spend your vacation all together, but if price really is an issue to them, you can help them come. It'd prevent abuse (and you wouldn't end up in paying more for helping all your friends) and reassure them on the fact that they can come anyway.
On easily splitting groceries expenses
On the point of your question when you say that you'd like to avoid having to deal with the groceries cost splitting all alone, I could suggest you use an application like Tricount. You just have to enter the amount spent for each thing and the number of people upon which to divide the spending, and the app automatically calculates how much every one owes to the others. I use this app on a daily basis with my partner, with whom I don't have a joined account. This way, if either one of us spends something that'll benefit for both, we know how much money we owe to the other.
If you want to use such app for monitoring your expenses during your trip, I suggest you tell your intentions to your friends at the beginning of your vacation, and that you show them how it is used. That way, you release them of the difficulty of getting to know "yet another app", and they know it's important to you that you all deal with the trip expenses in a tracable way. I discovered that way on a road trip with 2 friends in 2016; it worked wonders.
If you're afraid they'll think you're stingy by doing so, just tell them you're doing this for simplicity. As a person who's extremely nice/generous and who don't mind spending for others, I know I struggled to remind my friends they owed me money. But if you tell them it is for simplicity, they'd be more likely to accept using an expense-tracking app. If you don't have a proof of how much you spent for the others, they're likely to forget at least a bit about it.
Enjoy your vacation!