I had a Right Mastectomy with Axillary clearance. I had 3 wound drains. They all came out prior to discharge from hospital. I stayed in hospital 6 nights, in a private hospital. The wound drains are annoying to get around with. My hospital provided a bag with a long strap to put over the shoulder or head. A shopping bag or pillowcase will do.
The other annoying thing was keeping all of my wound site dry, as was my surgeons preference. That was until 2 week post op checkup. Luckily I have a shower with a hand piece, so I could shower lower parts carefully. Front opening PJ's were ideal for me.
I had no difficulties with my range of motion, of my arm. You will be given exercises, and should do them. My level of pain was very manageable, but everyone is different. Hopefully you will have some choices. Usually regular Paracetamol is recommended and stronger stuff as needed. You will probably need to ask for these, and will probably need some. I had and have some areas of numbness or abnormal sensation in my upper arm and side of chest. I still have this, to a lesser degree 3 years later. I don't know if the happens, if you do not have axillary clearance.
The other thing that may happen is a seroma (a collection of serous fluid) which may need draining after hospital discharge.
I recommend taking your own pillow. If you go to the room you will stay in Post Op, take the time to put things near you that you may want. Perhaps chargers, notepad and pen, book etc. Toiletries and fresh undies in the second draw or just be familiar where they are. If you go to your room only post op, maybe a relative or friend could do this for you.
I don't know if everyone gets their first mastectomy bra and soft prosthesis in hospital, but I was given this before discharge from hospital. Provided by a charity, but I cannot remember who.
You could make some soups or meals to freeze for after surgery. Accept any offers of help.
All the best. I hope it goes well for you.