If you are seeing Mulhall, you will be talking to the one who knows those answers best for your situation. You are asking the guys on the trail the questions you need to direct to the guru at the top.
But some experience - the time of the erection depends totally on how well you react to the doseage of the injectable (bimix, trimix, etc). Some of us get get mileage out of low doses. What gets a 20% for some guys here would send me to the ER. That is why the doc will have you do the first injection, and possibly more, under supervision.
Some complain of urine. Has happened rarely to me, often to others who have mentioned it.
Pain? Some for me, more if you do it wrong (hit a nerve, for example - that is not fun). Manageable? Yes. There is a pain vs. benefit tradeoff you will have to balance.
Is it "as good" as pre-surgery? For me, no. Things are different now. Does it last longer? Yes. Is a 2 hour woodie fun? Only if your partner can deal with it. Not all women want to have that around for hours. It is said that some do, but I don't seem to ever meet them
Needle size is up to you and your uro. I use 30ga. Others use thinner (bigger numbers are thinner - use a 27ga and you may never try again), but there starts to be a risk of breaking them when too thin, which is not a good idea.