So for this achievement you need to buy every mower. Meaning you need to have the Moderna Estate to begin with. This HQ has unlimited bays, which you need because you have to have every mower at once in your garage not buy them once and upgrade for better parts.
So what I did was contract 3x the 50k loan. That gave me 150k available at once. I then bought every mower.
Once the achievement popped, without any delay I might add, I quit the game from the dashboard, thus preventing the game to save.
Also : the Patterson RO-A and the SCAG V-Ride II 52" might now be required for this as they were added after launch so they're not technically original mowers. I didn't test this and didn't need to with 150k but it may be worth mentioning.
List of original mowers :
Stiga Estate 2084 H
Knight OFS1
Stiga Tornado 6108 HW
Patterson ZTA1
SCAG Patriot 52"
Toro Z Master 2000
Stiga Park Pro 540 IX
SCAG Turf Tiger II
Knight OFT
Toro Z master 7500-D
Toro ProLine H800
Toro Groundmaster 3300
These got me the achievement. So any mower, blowers, strimmers they add after on will not be required for the particular achievement.
Happy hunting all