First and second gear clunking under load

More than likely this is going to be related to the clutch that is failing or the slave cylinder is leaking not fully releasing the clutch.

Here are a few guides that will help with this:

https://www.2carpros.com/articles/clutch-doesnt-work-sometimes-or-not-at-all

https://www.2carpros.com/articles/clutch-symptoms

https://www.2carpros.com/articles/how-to-replace-a-clutch

https://www.2carpros.com/articles/throw-out-bearing-replacement

Basically, since this is a new to your vehicle, I would suggest we just go ahead and replace the clutch as that will solve this issue and your clutch is a good thing to start fresh with.

We do not have a manual on this vehicle so we are not able to provide the specific steps, but these guides above will surely get you close.

Please let me know what questions you have.

Thanks