the real estate agent is suggesting you paint your walls for 2 main reasons,
1) dark and bold colors can make a room seem smaller and neutral colors can make it feel larger.
2) because they think future buyers wont like dark & bold colors when looking for a house and that having neutral colors throughout your house will help potential buyers envision their things in the house easier.
while this can be true its not always the case, most good homebuyers know a wall can be easily painted and are looking for more important features in your house then paint colors on the wall. but it may help for some.
I would suggest re-painting the burgundy wall in the one room, and the multi-color room and leaving the rest.
when picking a color pick something neutral & light, but that also matches with the carpet & trim work of the room.
I suggest using a good quality white primer over the dark colors then after that dries completely do the final finished colors.
as for the gloss, semi-gloss, satin, flat: choices. it comes down to easy of cleaning & wiping down the walls.
gloss is the easiest to clean & flat almost impossible.
kitchens should use gloss for easy of cleaning. hallways and kids rooms semi-gloss(still easy to clean, but not as shiny) then I personally prefer the satin for any areas you don't have to worry about dirty fingerprints leaving smudges on the walls.
listen to the real estate agent to a point, just remember they want the house as great as possible and don't care what it will cost you in time and material, they only care about the finished product looks like. I've seen real-estate agents suggest hugely expensive changes to make something look a tiny bit better that don't really add any value. they don't always care about you because any improvements may help them sell easier meaning they get the commission into their bank acct. but the expense to fix it comes out of your bank acct, not theirs.
so don't blindly follow expensive suggestions they make.