As of today, you no longer need a certain rank to purchase any heist vehicle/clothing, as well as Pegasus vehicles.
Source - https://support.rockstargames.com/hc/en-us/articles/115004482908Heist vehicles now have a ‘Buy It Now’ This option works similarly to that offered with the Import/Export Special Vehicles. If players do not want to complete each vehicle’s associated Heist and wish to buy the vehicles immediately, they can use the new “Buy it Now” option and purchase the vehicle at an increased price. This includes the following vehicles:
Karin Kuruma
Karin Kuruma (Armored)
Lampadati Casco
Jobuilt Velum
Prison Bus
Mammoth Hydra
Valkyrie
HVY Insurgent
HVY Insurgent Pick-Up
Nagasaki Dinghy
Karin Technical
Maibatsu Mule
Brute Boxville
Savage
Declasse Gang Burrito
Heist equipment is now no longer locked behind the requirement of completing each item’s relevant Heist. This includes the following gear available for purchase at Ammu-Nation:
Rebreather
Night Vision
LCD Earpiece
The following vehicles are no longer locked behind Rank requirements:
Rhino Tank
Annihilator Helicopter
Buzzard Helicopter