Hi
@Kenny_ and welcome to linux.org

. I am from DownUnder (Australia) where it is lunchtime Wednesday.
I just have to do a few things and I'll be back in about 45 mins, likely.
Are you writing from the 'buntu lappie at the moment? If so, you could give us the output from Terminal of
and that will tell us what we could use with the lappie's specs.
Have you downloaded a Mint .iso?
Do you know you have to
burn it to a USB stick or DVD, not just copy it - special software has to be used, but that is available if your Ubuntu is up and running.
And do you know there is a newer version, v19. Tara?
is there any procedures i should do to make it more trustworthy and safe (in terms of it being a second hand machine)?
Not really, unless you really want to go the extra miles. Mint uses the Ubiquity Installer developed by Ubuntu, and in it you will get a choice to erase disk and install Mint. That is likely sufficient.
When Mint is installed, we can tell you how to enable your Firewall, and with one on the OS and one at your Router, you are very safe.
There is software we could use first to write zeroes to the HDD first and then install.
Think about it and I'll be back asap
Cheers
Chris Turner
wizardfromoz