Welcome to the sport
The Pulse R72 is actually a nice starter gun for a new young player so I'd say it was a good investment- I have one of these and have given 3 away as gifts to people who do lots of CQB back in Alberta. For about $100-150 in upgrades you could put in a JG metal mechbox and a large battery with a new motor so in terms of a 'kids' gun that can eventually be upgraded it is a good purchase in that regard. It is mostly compatible with TM G3 versions (things like hop-up and mags and such). I wouldnt use it at -15 though as the stock R72 has plastic gears and though they stand up fine in warmer weather they are more prone to break at cold weather temps. The hi-cap mag with my R72 fires about 30 BBs before I need to fully wind it again so yes you need to wind and wind or else get some other midcap mags (I use some 110round G3 mags by King Arms and they work though they need some tweaking to do so perfectly). Best thing about the R72 is that for CQB you fire around 220fps with a .2 bio BB so no one is going to get bloody welts as they learn
The DPMS is also an 'affordable' starter gun but in terms of upgrades and such it is less useful than the R72- the mags are not TM compatible and the receiver is not like most (TM is a company that makes several airsoft guns and in general people look for compatibility so they can repair and upgrade the guns). Its a good starter gun but it wont go very far in terms of upgrades and such. I dont own one but have seen them local and they tend to get dumped pretty fast as the players upgrade to guns that have a longer life to them.
At -15 your batteries will crap out much sooner- we did a day at -10 here and by hour 4 even people with the large batteries were noticing that they were losing power faster than on warm days.
As for the future purchases, if you are going to get into the sport heavy when your sons are of age then it is worth getting AV at some point as you do get a good selection of guns to choose from. If you are going to just stay on the fringe then there are a few Can retailers who have some lower end guns of note (the Kraken is always a fav) but keep in mind that often you get what you pay for and often low end guns need some guntech loving to keep working.