You have two choices whether you want to hire an appraiser to appraise what the lot would be worth and get estimates from contractors to built new or get estimates to rehab. You really should get a buyer agent who is well experienced in foreclosures iwho can guide you through this. You can usually negotiate it so it does not cost you anything out of pocket as the buyer agent gets paid an amount listed in MLS by the listing agent at closing. This way you have someone looking out for you and assisting you through each step of the buying process. good Luck with your purchase.