50 nights with Marriott will get you lounge access, both with the existing program and the new one. If you're US based, you can get the credit card for 15 nights and then you'll just need 35 each year.

For this year only, you can still qualify for SPG platinum with 25 stays, which some may find easier than 50 nights, and that will match to the new Marriott 50 night level and get you lounge access thru early 2020. But stay-based status goes away once the programs merge after this year.

Edit to add - welcome to Flyertalk. Stick around here, read and learn, and there's no need to pay anyone for advice.