Welcome to the forum However, you may not get many replies to this as there are very few people around these parts nowadays
First thing I would say is to get the correct sized wheels on it - the 160 was factory fitted with 17" 'Straights' with 205/45/17 tyres. The 16" Hairpins that are fitted to yours were the standard fit for the lesser models of ZR (but may have been retro fitted to yours for the sake of comfort). There are lots of styling/trim changes that you can make - too many to list really, but a browse around this and other MG Rover forums (especially threads from several years ago) should yield a few ideas. You could think about lowering it (there are several options with regard to lowering springs), and perhaps fitting a better flowing stainless exhaust system. A common upgrade to the ZR is to replace the Matrix seats with the Monaco type, but as the 160 is fitted with Monacos as standard, this is less of an option (if yours has the grey leather inserts in the bolsters, you could perhaps look for a coloured set, or possibly the harder to find Alcantara or full black leather seat facings).
Performance wise, there isn't much that you can do to the 160 (short of changing the engine for a more powerful one), and the little that can be acheived comes at a substantial price. Of course, there's nothing wrong with standard
Looks nice good performance can be got out of the VVC engine but you will lose drivability only option really is a full exhaust system, Piper cams, set of throttle bodies and a after market ECU so you'll need around 2.5-3k easy. Failing that a remap from K Maps is about your only choice but won't make that much difference really