Just forget about getting a spring one and go with an extremely long barreled AEG such as the M15, AUG, G36, SL8, SL9, M14, etc. This way you will not have to worry constantly about being out gunned by opposing forces, but you will still have a formidable long-ranged gun. If you're looking for something more silent, then look into the G36, SL8, SL9, and AUG as the have fiberglass/ABS plastic mixes that absorb sound better than metal bodied guns. Classic Army Even sells the 'CA8-2' which is a completely upgraded AEG minus the piston and spring.
It's just a suggestion. If you're set on the M24, it's a great replica. As for upgrades, the M24 can take a new spring, spring guide, piston, or if you want to avoid hard installation, they have drop in bolts which you just trade out for your stock bolt and it either makes it more powerful, or allows the use of gas. (Haven't seen a gas converted M24, but I've been told it's a good idea to avoid them).