Well your Dad's "M" is not worth $3500. It takes a one with real good tires and good sheet metal to break $1500 these days. Many don't break a Grand. So he is not going to get his price anytime soon.
Like the others have stated go talk to him first. Tell him you really want to buy his "M" but just don't have the extra money right now. See if he will put a more reasonable price on it. Then GET the money gathered up. The amount should not be that hard to come up with these days. Even if you have to take out a short term loan to get it.
As far as how your siblings will look at it. Well I have found that the siblings are usually driven by the "out laws"/spouses. They do not have nay skin in the game and make unreasonable demands. So if you get it bought have a clear bill of sale witnessed by some non family members. Even then be ready for trouble. People fight like dogs over every little nickle.
I have got two Aunts that I have not talked to for over 30 years because of the way they acted when my Grand Father died. They came in like Hyenas wanting the clear blue sky and lotto money. They did not get a dime. Grand Dad had told them when he gave them money for their houses that was their inheritance. It took a three year court fight to get the point across.