Question about Rollover 401K Setup

[Kin Lay]

Hi everyone, I quit my job last year around May 2019 to do a coding bootcamp to change career. I had around 40K in Fidelity 401K account and I rollover-ed to TDAmeritrade when I left the company. The current account type where the cash is at is "Rollover 401K". I thought about transferring the money to a self-direct IRA to buy rental property at auction. However, I don't think the amount I have can buy anything worthy, at least not enough for rehab and other expenses. With the recent fallen stock market due to corona-virus, I am thinking of using the money to invest in stock instead. Should I buy stock directly through my "Rollover 401K" account? Or should I open a new account type called "Roth IRA" with TDAmeritrade and transfer the "Rollover 401K" money and buy stock in that new account? Any other suggestion? I assume if I transfer from the "Rollover 401K" to a new "Roth IRA", I would have to pay tax on it right?