In 40k I do Tau, Orks, and Black Templar. To be honest if you want a *lot* of painting opportunity, do Orks, not Guard. You can go absolutely nuts on every and any models and when put together into a battlegroup they'll look fantastic, pretty much no matter what you do to them. You can also freely mod and paint to your heart's content because Orks are all about breaking the rules...thereby making it kind of the rule to break a few conventions with them. The Guard are a great army but a little bland IMO most of the time whereas Orks have many of their bonuses (painting and modeling-wise) but are more open to personal interpretation. We even have an "albino" Ork (he's red with green eyes).
My Tau are characterful though and pretty. Eldar are great to paint, too, but don't discount some of the things you can do with Tau painting-wise. Those Crisis Suits and tanks are very easy to modify and give you nice, big surfaces with which to experiment upon without too much frustration. Gaming-wise there's still plenty of punch in the Tau but we could use an update to help out a bit with the faster game that is 5th edition sooner rather than later. It's an unforgiving army (you *cannot* screw up with it) but you can punish people horribly with a little finesse.
The big thing about Tau is not that they're bad at close combat but rather that most of their weaponry is medium-range. This requires you to be mobile, much moreso than many other armies, because it's mobility on a combined-arms scale. You have to constantly pull, push, and tease your opponent into medium range where you can bring *all* of your firepower to bear. This means using railguns if they get too far, running if they get too close, and once they're in medium range knocking the living stuffing out of them with plasma cannons, railguns, pulse rifles, SMS systems, burst cannons...*everything.*
I find it very enjoyable but it's not for everyone. Eldar have a few more options, but I don't feel as much satisfaction with some of their stuff. Orks...oh hell it doesn't matter *how* you play them, I can guarantee a good time every time no matter what happens.