No Mortgage Needs To Sell

Seller is asking 139,900 and needs to sell because they are moving. They own house free and clear. CMA shows a value of 145,000. I am offering 125,000 and planning on Lease/option at 150,000 with 10,000 down. I can afford to buy house outright but would have to declare it a investment property and put 10% down. I can do it but I'd prefer to keep this cash for other deals. What are my options? Second, do I have to worry about two vastly different appraisel values assuming my tenant/buyers can get approved within six months of my purchase? Any help would be appreciated CK :-?