Depends on your budget, really.
A new headunit will improve sound quality, for sure, though not necessarily improve bass dramatically. I've yet to see a one fitted into an update 156 that looks particularly integrated, so you can't expect too much. You will want to add a dash kit fascia to fill the gap around the new headunit, but even these can look a little odd, and need trimming or respraying for best fitment. No big deal for an experienced installer.
An aftermarket headunit will make it easier/better to add an amplifier, because it will have RCA preouts for RCA cables feeding the audio to an amplifier. The factory headunit has no preouts; you would need to utilise the speakerwire outputs, but the signal is not as good quality as preouts. From the speakerwire outputs, you will need to use an LOC (line output converter) that drops the signal power down from high-level to low-level, to suit the RCA inputs on an amplifier. These are easy to fit and buy, though go with a decent quality product recommended by a good specialist ICE store (cheap ones are prone to interference; so buy what the store recommends, especially cos they should exchange it for something else if there's a problem).
Your ideal budget setup might include a new headunit, with fascia kit and steering wheel control interface; plus a 2-channel amp to run a sub; plus a set of 6.5" 2-way component speakers for the front doors running off the headunit. Or have a 4-channel amp to run the speakers and sub. Or expand from there. All dependent on the budget.
GBP$200-300 would be a good starting point for a good headunit, plus the fascia/steering interface. Then go from there.