An upgrade is an upgrade no matter the value of it
. You made a good choice, plus if you dont like the P280 in a few months time you can always sell this one and get the ghost.
Mods wise, im not sure without having a proper look at the cases interior. You may not need to do anything to it at all but if you're like me then you'll be finding something to tinker even if its perfect lol.
Edit: Just had a look at the interior and i can see the hard drive cage is fixed and built in unlike the older generation where it is a removable cage. I could see this as being abit of a problem if you're wanting to fit a radiator there... space wise. If it was upto me i would think about removing that section or removing some of it so i could mount a 240 rad. The top looks like it can only mount a 240 as well but again with some modding you could easily mount a 360 up there if you drilled in some custom mounting points (replace the roof with mesh would be the easiest and probably the simplest method). I would also make up a floor to seperate the psu area and the motherboard area. Again this can be easily achieved with a piece of smoked perspex or equivalent to your desired look and since you mount your psu upside down with the fan facing down anyways you could rest the perspex ontop of the psu and screw in the other side to the motherboard tray.
Another simple mod would be a nice case window, something that would just show off the nice bits of your system. I'm guessin the side panel on these are made of half plastic and aluminium so it would be best to take it to a CNC machining company or even a perspex/plastics company to cut and make you a window. The place i used charged me £50 for the whole lot, not bad considering the work needed doing to it and the smooth edging
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That's a quick run down for you lol, i'll think of more im sure