Selection of tires
A tough question. It depends on your requirements, driving habits and, last but not least, the amount of money you are ready to spend. Many of us find Michelin tires to outperform other brands mainly because their different, softer composition is better suited to the suspension and riding characteristics of Citroëns. I also found them to last much longer than, for instance, the Pirelli tires of the same price and performance class I had on the car before. But all that having been said, you will find other BXers who left Michelin for another make and don't regret their choice.
My personal opinion is that unless you want to exploit some special sporting characteristics of other tires or their different grip on dry or wet road surfaces (and you have to experience this for yourself, handling under different, sometimes extreme conditions is not something that could be described exactly by words), you should better stick to Michelin if you can afford it. And don't forget that cheap tires are generally more expensive in the long run as you have to buy a new set much more often. True, you'll have to pay in installments rather than all at once...
And then comes the evergreen debate of tire sizes. I share the opinion of Kjell T Svindland ("Changing to wider wheels for cosmetic reasons is generally a bad idea. Citroën engineers know what they do when they select a certain tire dimension for a certain car, and going up even as little as 10 mm on the width may deteriorate the handling and directional stability on bad roads."), Malcolm Gray-Stephens ("The trouble with wider tires is the unsprung weight; as the tires are heavier the ratio of the car on springs to wheel weight becomes worse; this gives poorer acceleration (inertia), poorer ride (but quicker response). And don't think that you can counteract the increased tire weight by using a light alloy wheel: most of them are not lighter but even havier than steel wheels. Wider tires can also disturb the steering weighting as the centre of the contact patch of the tire moves in or out relative to the pivot centre of the hub. It also results in increased wind resistance.") and Željko Nastasić ("Grip is proportional to drag which is proportional to pressure. On slippery surfaces, wider tires actually offer lower grip because the pressure is lower: the weight on the tire is distributed over a bigger area. Also, the tire profile and the height (tire outer diameter versus wheel diameter) influence comfort greatly. For instance, the difference between 195/15-60 and 195/15-65 on an XM is almost too big to believe: the slightly higher profile offers a much better ride comfort, but admittedly slightly worse handling in extremes (in tight curves, traversed at highly excessive speeds courtesy of hydractive suspension, you can actually hear the rear tires bending and producing a flop-flop-like noise as well as feel it in the movements of the tail of the vehicle").
However, others quote their different experience: Geir Korneliussen: "With a slight modification of the rear wheel arch wider tires can be fitted to the BX. On 14" I have tried 195/60 and it gave a nice smooth ride comfort. Today I use Enkei Sport Racing 7"×16" wheels with Michelin XGTV 215/45 VR 16 tires. This combination gives my car extremely precise handling and a roadhold my friends envy."