Things I am not crazy about is the paddle display and the need to replace batteries as you advance in your training. Also making upgrades like BM counterlungs and inverting the cylinders are not cheap when going through Poseidon.
The paddle is a little annoying once you get on the helium but it can be swapped for an M28 computer easily (on the Se7en). The battery issue is an interesting one. Originally the batteries were linked to cert limits, as a requirement by agencies. They would ship you a unit, but the battery would be sent to your instructor, thus reducing people diving beyond their cert limits without training. In theory.
That process is now gone, you can order whatever battery you want with the unit. On a Mk6 it makes sense to get a 60m trimix battery since thats the unit limit, on a Seven get a black Deep battery which is unlimited.
That sucks for those who got batteries back in the day but it is no longer an issue.
The BM lungs cost the same as the FM if you are buying a new unit and its a checkbox option when ordering. The inversion kit can be built yourself a bit cheaper but its around 100 USD from Poseidon, last time I checked. If you buy a tech Se7en new now, it will come ready for hypoxic dives out of the box (BP/W, deep battery, inversion kits, M28 etc)
That is the way I would go if I KNEW I would be ending up Mod3. If you want to dip your toes in, get a Se7en Sport, will be upgradable for around the same cost but the investment is lower while you are unsure.
If you PM me I can give you a line on some savings for upgrades etc.