Why 2 12 oz tanks in a LP machine?
24Oz is 380 liters of CO2 @1 bar. Each action of your flipper uses 4 liters , max.

The cheapest useable escs are third hand electronises.
Best bet, the BotBitz 30A.
Then you can look at IBF 30A 2 channel speedo, a very nice piece of kit, but with £100 I guess above your budget. Also, not available second hand. The teams using those swear by it.

But best Bang for the Buck, TZ85's. Less cheap, but there isn't a featherweight brushed motor that can't be controlled, inside the voltage limits.

5mm ali.... depends on quality. If it's 2000 T1, that's just a tad tougher than cold butter, 7075T6 is almost as strong as steel.
But for most purposes in non spinner combat, yes, it will do. On the other hand, 10mm HDPE does the job as well, and weights half of 5mm ali.