Weight is not a problem in the Bell 47. Depending on the model as long as the two crew weigh less than 550 pounds together away you go. That is with full fuel too. The Robinson is much more weight critical.
Regarding CFI's. I own Dutch Country Helicopters in Pennsylvania. I employ American and British instructors. I get resumes through the post all the time from European pilots. They usually follow the same path. They have trained somewhere in Florida or California and want a job with me with around 150 - 200 hours. The answer is no. To be honest it is a bit of a cheek to give their money to another flight school and then ask me for a job. If you come to me and train for your licence I will probably give you a job as a CFI, if you fit in. You will get at least 1000 hours in a couple of years.
Finally, a load of rubbish has been talked about the cost in the USA. At the moment we charge $200 per hour, all in. That is £109 at todays rate. A private will cost you 40 x 200 or £4371. A commercial from scratch would be 150 hours x 200 or £16,393. If you have fixed wing time (100 hours) that is reduced to 50 hours in a helicopter for a commercial or £5464.
You don't have to convert an FAA licence to fly G registered helicopters. It is an ICAO licence. Most flight schools in the UK will tell you that but it is untrue. The JAR compliant licence is the way to go. You can then fly anywhere in the world unlike the JAA licence.
This is not an advert, I hope this helps.