My preferred method is via ACH transfer. 

No, all of my tenants pay thru ACH (automated clearing house).  

I have seen on the forums if money was sent via credit card/ach a W-9 from the vendor is not needed. 

I am receiving all rent payments through ACH so rent collection is automatic. 

The one main theme I have seen over the years and in my current role as a funder of these assets in the wholesale condition before they are bought by out of state folks.. is that OOS folks really need to concentrate on the better properties in a given area.. where I see heart ache is when they try to buy super low value homes IE the cheapest homes on the market in a given MSA.. not realizing that the tenant is also the hardest to manage.. just moving up 20k on the purchase will put them in a much better position to have a decent tenant and in an area that is not so tough to own in. 

ACH is freeTakes about 5-7 days for the funds to hit your bank accountExcelI use an excel spreadsheet to track my check register.

Risk is the likelihood of not aching your goal.  

If you are worried about someone trying to muscle you out of your deal or out bid you after your seller signs it you can have them sign it with a notary and have that purchase agreement recorded at the county recorders which would cloud title.

After you know all those laws, I would talk with the seller about notifying the tenant that you will only (this can be done through the contract) be purchasing the property if they evict the tenant in the time period of escrow, and the unit needs to be investigated while in escrow, or you want a reduction in price due to the lack of knowledge of that unit.I have inherited a tenant and California is way different and more strict with tenant/landlord laws than most states, but luckily the tenant left without too much muscle (figuratively speaking).

Of course, sites like Cozy can take credit cards and ACH on behalf of you.