The general consensus is that if you decide to lengthen your tibias you should do externals and if you want to lengthen your femurs, then internals. Femurs allow for more lengthening - usually doctors say that 7-8 cm is considered safe for femural lengthening, although based on personal preferences and expectations, you might not like your final proportions. Tibias on the other hand are considered riskier because the bone is smaller to begin with. Thus, doctors do not recommend that 6 cm be exceeded, although people have reported unproblematic lengthening up to 8 cm.
As for a doctor, it depends on your budget. The doctors in the US and Western Europe are considered to be the most experienced and trustworthy. People generally agree that doctors in Eastern Europe (mainly Russia, Ukraine, Serbia) are next ''tier'' so to say. Dr. Franz from South Africa is highly regarded and also preferred option for people with limited funds. Some doctors from South Korea offer relatively affordable internal options in a country with high medical and ethical standards. Finally, people with the least amount of funds they are ready to spend go to India. Results are experiences are mixed. General consensus is that you should avoid Dr. Sarin at all costs and that Dr. Parihar is really really good, although prise-wise he is really expensive for Indian standards.
I hope I summerized it well. Good luck !