i dont have a limit to my budget but i just want to increase the power in a sensible fashion without making drastic changes to the internals.
The most sensible fashion I know is the Metric Mechanic unit... Especially if You are not real tuner itself. It is the most complex and proven way of increasing power to M42.
W/O drastic changes to the internals -> You can put there chip allowing higher revs /7000rpm/ -> and this causes +13 up to +17Hp's /depending on the gasoline type it is designed for/ Put there cone filter +5Hp's and lighten Your flywheel. This is the most sensible way of getting some Hp's to the engine. Changing the exhaust will change the sound, will maybe give it a little more flow at the end, but You will lose in the midrange.
So w/o rebuilding the unit, You can achieve more agressive /due to light flywheel/ and stronger /about 20-25Hp's/ unit. Changing the cams won't do any magic-> and surely without changing compression ratio, head flow, and lightening inside parts it is simply not calculating.
You can change the transmission to short one /for example 5spd getrag 225/ with 5th 1:1 but this will cause lower maximum speed, and You can surely change Your differential -for example to 4.45 unit. This will again shorten Your max speed but will transmit more wheel torque at lower speeds. But that's all.
For serious powergains go MM engine, for easier but less driveable -> go supercharged and for the most efficient improvement go TURBO! And Take the advantage of having 400+Hp's under the hood

Of corse every one of theese engines appear differently, the most streetable option is N/A engine than a supercharged , but due to the LAG turbocharging the engine is not always liked by drivers. But it surely gives the most powerful unit.
My aid -> unless You strongly feel the engines by Yourself - keep it simple ;-)