I would strongly advice against an FHA rehab loan for a few reasons. #1) The FHA program has greatly increased its mortgage insurance premium, I think you would be hard pressed to pay less than $100/mo in Mortgage insurance which is just money down the drain #2) there is a huge up-front mortgage insurance premium (MIP). If you are seriously considering that option get a GFE/TIL early on in the process so you know all of the fees you are looking at - they are extremely substantial.
Paying for it in cash would be the easiest option - you could potentially take out a loan on it for 80%LTV or less afterwards if necessary. However, be aware that most banks will be looking at your total income (not including any rental income you haven't had for 2 years!) when making that loan. I guess, being conservative, I would take out the HELOC and plan not to use it, but at least have it if you find out that there is some huge repair that you missed in your due diligence.