You say your dad has been talking about making more happy memories there, while at the same time your mom is talking about selling. Are your dad and mom not even talking to each other about this property? It sounds like they are not on the same page at all -- as if maybe she has this fixed idea that they will definitely sell, while he is blithely blind to what she is saying. The two of them must first come to an agreement over the property's future before you and your husband can even think about buying it from them. Talk to your dad and see if you can wake him up to the fact your mom seems serious about selling. I would bet that she hasn't consulted anyone about really putting it on the market or finding out a realistic market price.
I would not pay any HOA fees, upkeep or anything else until mom and dad are on the same page. If they do indeed sell to someone else, you will just be out of the money you paid for those fees (though it is great of you to offer this and is the right thing to do!). If you really are that attached to the property, then do try to buy it; your dad may be able to convince your mom to let your family buy it.
I would never, never co-own it with them, however; that would be a recipe for a huge family rift if later Mom goes back to "I insist we sell to someone else" mode. Please believe me and take it from this stranger -- I have never, ever seen any good come of family members co-owning homes; rifts and strife have always been the outcome.
The idea of your buying it may upset her because she still uses the property and she fears you will be cold to the idea of her and your dad going there if you are the owners. (Even if you swear she can use it until she dies, she may still feel this way -- you can't read or interpret her feelings, only her actions.) She may not be saying this but using her "I want $X" as a way to shut down your discussion of a purchase by you. You might ask her, "Mom, are you concerned that if we buy the house, you and dad won't be welcome to use it after that?" I bet she'll say "No" and insist she wants the money. But it's worth asking.
Also, you say your folks are OK financially but there may be financial problems there of which you are not aware, which could make her worried about having enough money as they age. Their generation was taught to keep money worries private even from their adult children, so dont' assume that they are doing fine; ask your dad frankly if there is an issue with needing money. (And hope he tells you the truth if there is.) Or it could be that though they are OK right now financially, she is just plain afraid of "what might happen if our investments go wrong, or if we can't sell our main home, or if our bank goes under...." etc. Older people worry about these things a lot, I find, and that may be behind some of her cold determination to sell the house eventually -- she sees it as a buffer against an old age of poverty (even if she's not poor now!).
Ultimately, too, prepare yourself emotionally to love the house now and lose it later. It's hard to give up a place with childhood memories but you will build your own with your young family wherever you are. Losing the house to some other buyer -- and being able to let it go gracefully and without rancor -- would sure beat a permanent rift with your parents over a piece of property, however wonderful that property may be.