I would not be able to carry the investment solely with my own finances (hence the partner), so it seems the bank would refuse to lend to me on that basis.Are partnership investments simply limited to commercial loans only? 

Small community banks or credit unions are going to be more flexible in their terms. 

The county development office would be a good source for permit and plat history to get a sense of quantity, and individual property cards (and recorded deed transfers) may give you a sense of price direction.

You should get tested for any blood related diseases that could have transferred to you, there are more than you think

The previous lease converted to a month-to-month and it transfers with the sale of the property.

Using a small bank is your best option.

Can anyone guide me on where to find this information on whether it's legal in Texas.Thanks Yes and No.Generally, you can do a Lease Option in Texas that is 6 months or shorter if it has a bank loan on it.

But I wouldn't think it to be a smart move on my part as a beginner to play around with swings in the market and bank on them.

So my offer was bank financing for 70%, seller financing for 20%, and 10% down. 

Not every bank likes development deals, but once you find the right bank they will become your partner in the deal.