I would do this every 12-15 months to build my portfolio.

They do just about every month or so investor calls and K1 has been prompt.  

It could cost only one month's cashflow.

Trying to figure out what I can/should expect to charge for 1)Purchase price; 2) Down payment and; 3) the spread per month if any on what I pay per month.

you need to own the land or have more experience. make 100 calls to land developers nationally. use linkedin and get some experience for a month. this is a self taught business. very specialized. 

Brandon Turner was on fire in 2020, 2021, and 2022, but I don't see that he's done anything for the last 18 months. 

Meaning you cannot just evict a tenant because their lease has expired.You will have to build sufficient evidence admissible in court and to the satisfaction of a judge to prove good cause to get a warrant of eviction.Many housing providers kept high risk tenants on month to month leases so that if the tenant stopped paying rent, you could simply give them a notice to vacate and recover possession of the rental unit that way.The reason for this methodology was because the HSTPA of 2019 passed in NYS made it extremely onerous and costly for housing providers to evict due to non payment.That method under good cause will be eliminated.So you can still keep tenants month to month but you will need to document the lease violation due to nonpayment rigorously.How many times and how late is “good” enough?

There are a couple of things that I would think about here, the timeframe you want to work with this agent (can be 1 month or 2 months or a longer time), whether you could do it for a specific area/city/town, or you could do it for a couple of properties you see.

I just don't want to waste my realtors time with backing out on 2 deals in 1 month

I always plan to have 6 months of holding costs available, for after I finish the flip, just in case I missed the market.If a lot of houses are sitting on the market for a long time, (can't sell) I'd definitely avoid it.