A remap by itself should get you there, outside of that you'd need induction kit and an exhaust which might get you close but I couldn't say for sure it would be a dead cert you'd get 150+bhp that way.
Remember, many of the components used on a 318 are the same as those used on a larger engined car, so it won't be pushing it too much.
Now - the remap getting 150bhp would be based on the engine being the more powerful 143bhp 2.0 N series, and not the M43 1.9 with 120bhp.
The 16 valve engines can trace their history back to the e30, which managed 140 bhp from 1.8 litres.
When the M44 came along, it was increased to 1.9 litres, but pinned at 140bhp.
The N series increased the size again to 2.0 litres but only a token increase in power so you can be sure it was never pushed to it's maximum.
Yes, the valvetronics do have problems, usually electrical from the valvetronic motor, wear on the eccentric valvetronic shaft, or the timing chain jumping teeth when the flimsy plastic guide snaps, but none of these will be under extra strain or wear from tuning.
If you are just looking for power, then your best bet is the remap.
An exhaust / backbox would cost similar money with less increases.
You've got the induction kit already, and the only other options are stuff like underdrive pulleys, fan delete and decats.