For me it would have been to stockpile RIF's from Taiwangun pre-BREXIT. Of course that doesn't apply to anyone coming in to the hobby now.
I would advise to quickly get it into your head, that what @Rogerborg says about all AEG's essentially doing the same thing is true and if you play mostly woodland, you're not going to get that much use out of a pistol. So when you get your defence, start with a budget to mid-range AEG, replace the inner barrel and hop rubber (learn how to do this yourself!)
Use that until you've been around a while and if at that point you're really craving the newest/best AEG that's fine, but keep your old one as a backup. For a sidearm, just get a mid-sized TM and don't pay it much mind after that.
That being said, it's also fine to get something for "the collection" but do it with acceptance that it's a luxury, don't try to convince yourself that you "need" it.