A. With regards to the duration of the traspaso, you will have to see for how long does the original contract of the tenant provide for, since you take over the lease with the same conditions as the original tenant.
Provided the original contract does not stipulate otherwise, you can sell later without any problem. You need to notify the lanlord within one month of the ´sale´. The owner is entitled to an increase of 20% of the monthly costs every time a traspaso takes place.
With regards to the last question, I could not tell you without going wrong. The price may depend in many factors such as type of business, annual turnover, location and obviously discretion of the landlord, since he puts the price. A reputable commercial estate agent should be able to guide you.