Tip make a save before customising the Stronghold incase the achievement doesnt pop first time.
The cheapest stronghold to fully upgrade is the Safeword
Stronghold unlocked part way through Act 2.
Safeword is located on the far right
island in the Morning Star 'Rosenoaks' region (It will be listed as a crib) and can be accessed from either the helipad or the garage, just head for the elevator to the 'Penthouse'.
Once inside the Penthouse press
to bring up the Crib Customisation menu and enter then enter the 'Stronghold Customisation' from here there are two upgrades which both need to be purchased.Upgrade 1 costs $20,000 and Upgrade 2 costs $40,000 so you will need a total of $60,000 to unlock the achievement.
Purchase both upgrades and the achievement will pop.
Here's a video demonstrating entering from the garage.