I have a house that I can sell to afford internal femurs or do you recommend me external tibias for a much lower price but way more uncomfortable?
Do tibias, try 7 or even 7,5 cm (3"") if it's with a good experienced doctor with a competent physiotherapy team, and things are going ok during recovery.
Don't sell the house.
"Best" tibias lengthening method is externals which is also the cheapest one!
Doing tibias furthermore has many other, even more important advantages (mainly with externals, nails in tibias are not as recommended as in femus), despite not being possibly the majority trend in this forum: its saffer(!): for example, if formed, blood clots are smaller, less chance of nerve injury, etc; less pain, less agressive and less risky for bigger tendons, nerves and muscles, etc.
However try to strengthen Achilles tendon even before surgery, be ready do a lot during physiotherapy and also be ready to do and pay for Achilles tendon lenghtening surgery if still needed.