Private contract for land purchase in installments

[freeflowwales]

Hi we are in Scotland and looking to buy a piece of land, can one have a private-style agreement drawn up, for a Private mortgage, where we pay half the property cost up front and the rest in monthly installments? The agreement must protect us and the seller equally, make the property legally ours, once all the money has been paid, but protect the seller against any legal liability in the meantime. Is that something a solicitor can do? Is it still legal to do?