First don't be confused with AS350 vs H125, they are the same thing. What is coming off the assemble line right now is an AS350B3E, however Airbus chose to change a lot of the model names a few years ago to line them up with their airplane division. The "AS" prefix is from Aerospatiale, the original french company who designed the AS350. There have been many versions of the AS350 over the years since first released in the 1970's. AS350B, AS350BB, AS350BA, AS350B1, AS350B2, AS350B3, AS350B3-2B1, and the latest version the AS350B3E. There are other versions too but just putting the B's to keep it simple.
Then you had Eurocopter who was the German side of the house coming from the old MBB which designed the BO105/BK117. Now Aerospatiale and MBB together became Eurocopter and any new products designed had an EC prefix, so the EC130, EC135, EC145 etc. Once everything became Airbus Helicopters they eventually went back and changed all aircraft that had an "EC" prefix to an H. So now we have the H130, H145 etc. Anything with an "AS" was left alone and considered a legacy aircraft but eventually they decided to change the AS to an H as well for the AS350 but only for the latest version coming off the line, the AS350B3E is now called the H125. If you have an old AS350B2, it is still an AS350B2. Its all just marketing to fall in line with the airplanes which are A320, A340 etc.
B2's had the 1D1 engine, the B3 and B3-2B1 had the 2B and 2B1 engine which were essentially the same except for the engine control changing from a DECU to FADEC and the change of the twist grip from a manual cable to just a big switch.
The B3E has the 2D engine which is an absolute beast, almost 1000hp vs appx 700hp for the B2, or 847hp for the B3 and B3-2B1.
The dual hydraulics were never an option for the B2, the hydraulic system from the AS355 was modified to go on the EC130 and then later brought over the AS350B3-2B1 line as an option. It's also the same FADEC design that came from the EC130 that made its way to the B3-2B1.
The VEMD was originally developed for the B3 and then eventually brought to the B2's. The first B2 with a VEMD was SN 4129 which was a 2006 AS350B2, that's when the change over occurred.
In the big picture the airframe has remained the same however there are always dozens and dozens of mods that are done to the airframe over the years. This year they are now being delivered with dual sliding doors standard. Before 2007 the aircraft came with no sliding doors but were options. Then the left sliding door was standard and a right slider an option for about 90-100K extra. Also the crash worth seats came in around 2007 as well. Last year there is now a new version of the crash worthy seats that has carbon fiber and new look.
For the dual hydraulic accidents there is a large amount of information out there you can read about but I'll give you some basics.
Dual hydraulic version will always keep the main rotor controls, collective and cyclic boosted. The pilot has no switch in the cockpit to turn the main rotor servos off. The cut off switch on the collective only stops hydraulic pressure flowing to the tail rotor servo which is an old school single channel servo. Guys were leaving the switch in the off position during startup/shutdown since the checklist had you turning the switch on and off to check its function. There was no indication visually in the cockpit that the switch was in the off position and pilots took off with unboosted pedals. If properly trained you can fly the aircraft with unboosted pedals without issue, however if you never had the chance to push on a flight control as hard as you have to with unboosted pedals on a B3 you would think the pedal would snap off. As a result pilot takes off, pedals are stiff, assumes its a stuck pedal and the ensuing spin ends up in a loss of control/crash.
They first changed the startup/shutdown checklist and moved the hydraulic check to after shutdown and removed the activation of the switch during it. Then they came out with a service bulletin, SB No. AS350-67.00.64 which added a second HYD light so HYD1 and HYD 2 and made it so that if the cut off switch is in the off position the HYD 2 would flash to let the pilot know the switch is in the wrong position. This is where we are at now with that. But ultimately poor pilot training with the hydraulic systems and then failure to follow the basic 2 step pickup which would have stopped accident chain is what happened. We teach hydraulic off training extensively and I've given the exact same failure as those three accidents to all our pilots including the smallest 115 lbs pilot we have and they have had zero issues in controlling the aircraft.
Any other questions you have on the AS350 series I will help you with as well.