It fine! we all need to start somewhere, welcome to the obsession-er i mean hobby! haha, and after the fruit has been crushed it's called must
Well I am not sure on a red plum wine, in my experience it makes a nice "White" wine. but as i think about this many red wines are said to have "Plum notes" so hey, why not!
if you are wanting a red style wine i would say go ahead and add the raisins, they will add body. what kind of grapes are you thinking of using for your added must? the type of grape you would want would be something that pairs with plums.
As for sugar, you should always go by your hydrometer, not by what is called for by a recipe, i try to shoot for a S.G. below 1.090 and above 1.080.
now onto the plums themselves. if i remember correctly victory is a purple plum that Canada released back in the 1980-90's. i think they are fairly good sized too. if yours aren't this or my memory is wrong feel free to correct me, i don't work with plums much.
Do you have a TA kit? i would test the acid of the juice of some of your plums and then use that to decide how much you want to have in your wine.