It is great that you are seeking help for your symptoms after all of these years! A urodynamic evaluation in which the pressures and sensations within your bladder while filling and when urinating will be measured as well as the speed and other characteristics of your urinating will be evaluated is definitely the proper way t o approach your issues. I trust that your urologist will also perform cystoscopy (looking inside your bladder and urethra to observe you internal anatomy). Certainly scarring within your urethra or abnormal anatomy with which you were born can cause your urinating symptoms.
Your problems with erections may or may not be related. This could be an emotional issue associated with your urinating symptoms or it could be related to an abnormality of the nerves that control your urinating.
Hopefully, your upcoming study will provide the answers.
S.A.Liroff, M.D.