On track to the E50M

Yes. I agree with advice Aschenblume gave you before that focusing on one line is key, and when these events happen, you can quickly research 2 or 3 tanks, depending on tier.
All of the tanks in the E50 line are fun to drive, as long as you realize that many of them are sniping, second-line mediums and not brawlers like the soviet line.

I suggest you sacrifice the gold for a few slots...you may want to keep the 30.02m.

The line is pretty good. The tier 6 is the traditional panther with the turret, hull and gun. It's a good all round support vehicle. The tier 7 Panther is mediocre because of its Damage output. Even though it has high penetration it's damage is quite low. It's a good sniper but not amazing. The armour is not bad as well and take hits nicely. The Panther II is a modified Panther with a better gun and slightly better armour. You shouldn't rely on it but it can do well. Again, a sniper. The tier 9/10 I'm currently working on myself as I'm only 20k away from the E-50. And boy I've heard amazing things when this tank is fully upgraded.

The tank line is a sniping line but the tier 10 can be with heavies. It's pretty well armoured and good speed but the problem is that they have the worst DPM out of the high tier mediums. They're also pretty large for mediums but I think that's where the 271mm or penetration comes in. Also, russian mediums can't ram tanks as good as the E-50m

Oh my...it's been ages since I played the tech tree Pz IV so I don't know how to answer. I just really enjoy the way the M plays. I just got the Panther the other day before this on-track was announced so I'm still on the stock package (stock package is slow).  Also, I love tier 6, so the M is effective enough to keep along with the other great tier 6 mediums (for me anyway).  Sorry again, but I'm not knowledgable enough to answer the "heavier" medium part of your question. I'm hoping to make it through both Panthers with this on-track and hopefully get the tier 9. Even with these on-tracks, I tend to get a little burned out with battle after battle in the same tank. 

If you really enjoy playing the E50 with your crew that has perks, is it worth pulling that crew into the E50M? Sounds like a stupid question, but from what I have read, I'm not sure I will enjoy the 50M as much as the Tier 9 E50. Would like to keep a tank I enjoy.

This is another line that I feel like I would really like. I probably won't be participating in the On-Track strictly because I have other lines that I am focusing on. I should learn to focus on specific lines and just bust them out, but I always play around. Either way, the E50M line seems fun, especially at tier 9 and 10.

This On Track has motivated me to finally grind this line. I had been putting it off for forever (since the OG 30.01H got zapped). Just unlocked the Panther II. First match was on Dragon Ridge. Took a hit in the engine. The stock engine. Climbing those hills with hamster wheel power seriously almost made me put down the controller. Used gold to fully upgrade, because screw one more painful match looking at that hideous stock turret.

The gun "upgrade" was a fun little surprise, too.