This is not a security deposit the property owner is asking.
It is really just a loop hole for property owners to get around the tenant laws. The first thing you need to know is that this is not a real rental agreement. Therefore none of the laws protecting tenants apply. The property owner legally doesn't have to fix or maintain the property at all like he would if you were normally renting it. What you are doing is loaning the property owner a lump sum of money and in return he is offering you the opportunity to hold value of his property (as is) as collateral and in place of a loan fee, or you are agreeing to take the loan fee for a (barter) reduction in the normal rental price of his property for the duration of the loan.
Normal rental agreements are open ended contracts and even after the original lease has expired as long as the tenant pays the rent, the tenant can stay indefinitely at the same fixed price of rent agreement until a new rental agreement is reached. This is not the same for what you are talking about.
I have done this kind of large sum contract thing for about the same kind of money myself. It worked out well, but there are risks. For the amount you are talking about, it can be a good deal if you wish to stay in the property for a quite a while. I lived in my apartment for 3 years before my money was returned and I then had to leave.
I would not suggest you pay any more than this amount. The issue is getting it back when you wish to leave. If you leave the property without the money in hand, you may never get your money back. I have friends who put down about 15000 and they didn't have to pay any rent at all, but when the contract was up the owner of the property couldn't return the money, so they had no choice but to stay and hold the man's property until he can pay. This is fine if you like the place, but it really sucks if you don't, or you need to move to another city or back to Europe/America. But, this can go the other way too. I know of a case where the property owner decided in the middle of the contract that he needed more money and when the first tenant who had lived there for years couldn't offer that money the property owner then offered the same property to another tenant who did pay it. The property owner then returned to the first tenant and gave back the money before it was due and told him he had to get his family out immediately (2 days). In the end it was a mad scramble to pack and find another place to go. For most people it is fine and they have no problems. But, you need to know that this is not what you think it is before you get into it.
Ask questions and find out why the property owner wants to do this rather than a regular rental agreement.
I hope you find what you need and it works out well in the end.