Not to be a dick but I'm a little concerned that you are asking for advice. This is a pretty simple install.

If you aren't going to do an exploratory visit then bring both lags and toggles with you. The back of the cabinet is either 3/4" particle board, 3/4" hardwood, or a 1/4" skin.

-If it's 3/4" hardwood, use screws wherever you want. I would pre-drill the holes so that you have more thread bite. Don't use too large of a drill bit.
-If it's 3/4" particle board, I'd use 4 large toggles as well as 4 lags pre-drilling like above. Toggles will ensure the screws will never fall out. The only reason I wouldn't just use lags is that with any type of moisture, the MDF (or particle board) could swell and pull out over time. Ask me how I know...
-If it's 1/4" skin, you have to go back to a stud. If the cabinet is pulled away from the back wall, then you will have to find an alternative.

The alternative would be to frame the section your TV would go. Use 2x4's. You could either frame it for the depth of an art arm or pull it forward and use a fixed mount. Use a 3/4" hardwood as the back wall the TV will mount to. That 3/4" hardwood will mount to your 2x4 framing. Stain the 3/4" hardwood accordingly.

After all of that, install new shelves below the TV so that everything looks like it was meant to be. If your wood working skills are up to the task, hire a carpenter or a cabinet guy. This is a super easy job for them.