I'm from the same world you are (I still play competitive Xball), here's some things I found in regards to your q's:
1. Not many people here (besides me) will know what an Ego is. Most of the stats you listed there can be changed and upgraded to what you need (just like pb). You can think of an M4 as a good starting gun. I say this because there are many upgrades and parts available for it. There are many different brands which make M4's. With it comes different quality. So basically without knowing your friend's "some type of M4" noone can make a determination until they know what brand it is, and what may have been upgraded with it.
2. Been answered by other posts
3. Only thing I'll add here is that you probably already have the necessary goggles from paintball to play. I would stick to paintball goggles because a lot of fields that allow airsoft to be played are paintball fields, which NEED paintball goggles due to insurance reasons. Airsoft goggles are not always covered by their type of insurance and hence are not always accepted at paintball fields.
4. Airsoft, I would say, is probably half the cost of speedball, depending on where you went for paintball. Let's use the Cage at Raiders for example. The Cage would require entry, which was usually $25 and a case of paint which was $35-50, this adds up to (lets say on the cheap side) to be about $60 for the day to play. To top it off, if scrims are going quick the day would last probably 3-4 hrs max for the entire case of paint.
In Airsoft, I would probably pay somewhere around $10-25 for entry for a scrim day, and for 2000 rounds of BB's I would pay somewhere around $10-20. Already its cheaper even if we compare the most expensive airsoft day to the cheapest pb day. To top it off, I would RARELY go over 2000 rounds in one day.
Also I'm not taking into account large scenario games, where the entry is usually more than scrim days, just like large scenarios in PB.
5. Depends on your style of play and the scenario of the game is. I would say it can be similar to playing back, front or mid in paintball. I play mid mainly in paintball carrying anywhere from 4-6 pods a game. In airsoft I would carry around 6-8 68rd mags. Most of the time I wouldn't go through all of them anyways.
Here's some other things to take note, if you're expecting airsoft guns to be like markers, you'd be in for a pleasant surprise. I know I can do a complete break down of an Ego (bolt, rammer, regs, and frame) and clean it in under 30 min. With an AEG there are more moving parts and it takes a lot more time (for me) to disassemble and reassemble all parts for my airsoft guns. If you weren't a techy with your marker, I wouldn't recommend tinkering and breaking down things yourself. Drop it off to a tech to look at and take the time to read up and learn some things