Difficult question, bit like how long is piece of string. Firstly you have to answer some questions.
First, what do want do with loco once you built, are going run it at a club or private track or is it going be a display model.
If you are going run it, are any issues about loading/unloading the loco. 5" locos tend to be heavy, unless very small models, will tend to weigh 50kg +, ie 2 man lift.
Depending on your engineering skills realistically you are looking at 5-10 year build, do you have any preferace of loco type, ie do want go for a protype design or a freelance design ?
Do you medical/physical problems, ie having a back problem might make driving a tender loco uncomfortable.
One piece of advice I can give is to join a club.
Hope this gives some food for thought.
Regards Luigi