Hate to say it, but I think you're getting bad advice... not here... but from your "tech". Who is this "tech" telling you this and that? Don't assume they are giving you good information. Any idiot can be a "tech", yourself included. Our advice is based off of what you provide, if you leave something out, our advice is skewed. So even our offerings can be bad advice if we don't know the whole story.
The key thing you are after is this:
A machine capable of running the old Adobe CS6 suite. It runs best under High Sierra. So you don't want a machine that doesn't run High Sierra. Mojave is still possible but honestly not a plus in your case for a number of reasons.
The next key thing you are after is cost:
You don't explicitly state what your maximum budget is to resolve you current situation. If you are really that strapped for money... I mean really strapped, there may be no solution that you can afford. We can't offer options without knowing your upper limit. Only you know that number.
Keep in mind, switching to different software other than what you are used to comes at a price (price of the software, learning curve to adjust to it, hurdles to undergo trying to convert formats, etc). Some people make such adjustments easily... for others it's like trying to learn Latin overnight. What impact will such a transition have on you? Is this software used primarily as a hobby or required for your livelihood? Down time impacts the bottom line.
So first, make a choice, it's the toughest one... Are you willing to up and learn entirely new software that may or may not import work you have done in the past? Yes or no.
The last key thing is what you are currently using now:
Tell us exactly what your existing hardware space is. What computer, make/model, what OS you are running, RAM, GPU, storage (size), peripherals that you have attached to it (make/model) and how it is attached (USB, Firewire, et al).
If we know your budget, your choice about abandoning CS6 cold turkey, and what you currently have now... we can give you constructive advice.
P.S. If someone is offering to charge you $90 (above the cost of the machine) to install Mojave on a used machine before you purchase it... you are almost certainly paying more than you should for the machine itself. Don't buy anything from them.
Here is an example of my current setup (for OP to see what kind of info we would like to see):
Mac Pro (Mid 2010)
3.33 GHz 6-Core Intel Xeon
6GB RAM
AMD Radeon R9 290X 3GB
3 Internal 1TB SATA HDD
1 Internal 4TB SATA HDD
27" Apple LED Cinema Display
Apple keyboard (USB)
Logitech M705 Marathon Mouse (Wireless/USB dongle)
HP LaserJet P2035n Printer (Ethernet)
Mojave Version 10.14.6