For the record, I just let my subscription time die, and here are the results...
They all still appear on my squad list, they're just greyed out and unselectable. It still reads them all as "MAX" Level, so presumably if I re-subscribe... they'll all just become re-selectable.
All the squad levels I gained from them are still there.
I bought one month of subscription time, made sure to earn my 170 gold every day (20 for consecutive logins (5 if you don't subscribe) and 150 from the wheel) I think I only missed one day of the wheel... I logged in, so my 20 gold bonus stayed, just a brain fart. forgot to do the wheel.
I also bought 1 card for 1500g since they were on sale at TRU.
The heroes I bought for gold, plus, the SHIELD Agent heroes, provided me just about enough levels to get all the way up to FF Spidey.
Your mileage may vary depending what heroes you buy with your gold (Expensive vs. Cheap. Some are worth +0. Some +3. Ya'know)
But give or take a hero or two... you should be able to complete all the currently available challenges with one month of subscription. Be sure you MAX your SHILED heroes though, for maximum squad level gains once your time dies. You can always continue leveling the gold heroes after the time dies. So give SHIELD heroes priority.
I had no idea letting agent status go would lock the agent only heroes. That is ridiculous. You spend money to be an agent and get the heroes but can't keep them unless you continue to pay?? That is a pathetic tactic for Gazillion... I can understand needing incentives to remain an agent (like 500 gold a month and a full gold wheel every day) but taking away something you bought seems kind of low.