*Well beauty is in the eye of the beholder, so it's all up to you. anything from a front lip to new rims will improve appearance.
*Changing the sound without needing a re-tune is as simple as changing the mufflers for more bruuum bruuuum. AVO or Xforce are popular choices.
If you want sssssssssssucky sucky induction sound a hi-flow intake filter and putting your window down is also a low cost option. Not going to go into the argument of oiled vs non-oiled filters here (coz i don't care). It's up to you. I use a K&N and clean my intake sensor once in a while for good measure, never had any issues. But it's your choice, no one else should give a shit what you choose!
And, of course, if you want yee 'olde bruuuum pshhhh sound change the BOV to a named brand (adjustable is nice).
* First thing to say about performance mods is what Benny always says, and for good reason... Get an engine health check FIRST to ensure you have a good platform to start with. if not correct any flaws!
That being said, all the 'usual' engine mods can be done to a boxer, just like any other car. BUT any engine mod that will give a noticeable performance increase will require a re-tune. So it's best to do your due diligence regarding the specifics of what, exactly, you want to achieve, what parts you need to do it and how to go about it.
e.g. TBE (if an N/A engine CBE), is a good and easy DIY place to start, but again, will require a tune.
if you're chasing a large increase with big HP and dyno figures, you will need to start changing engine internals and eventually you'll find the block and transmission etc etc are the limiting factors.
There is already a lot of info in here (forum) and 'out there' regarding gen-5 performance mods, you just need to take the time to understand them. Then go go go! $$$$$$$$
*Handling mods are pretty straight forward. Start with good tyres. Ensure your bushes are in good condition, particularly the LCA's. If replacing them, stock bushes are squishy squishy but give a comfortable ride. Poly's are like morning wood, increase drive feel but will also harden the ride. There are also ones in between such as group-n.
Then sway bars and chassis bars help with body roll and chassis flex.
And round it off with some coil overs. If in OZ, Shockworks are a good choice.
There are many options and combinations to choose from. Your first step should be, getting specific about what you want.