Welcome aboard
To play on W7 you need a copy of SH3 without starforce. Also XP while still good really is not advised from an IT standpoint. W7 64bit is the way to go as it is more current.
As for what system to get modern laptops can play SH3 as the integrated graphics cards have received major upgrades in the past 10 years. My macbook pro handles the major graphics mods including MEP4 without even flinching. Desktops will likely be the same way you can should be able to get one off the shelf without building it yourself and run the game (though they all have W8 on them now). I recommend getting one built for the simple reason you can make a computer that will hold up to newer games for about 4 years were as the "off shelf" will be out of date in about a year and you can put whatever OS you want on it (minus mac) without much fuss.