As a newbie in trials I'd like to ask you what are the weakest points in that bike. I bought one in good condition but its good to know what should I take care of. Thanks

Review the Beta forum. There are numerous articles on the Beta Rev 3 series. Pay particular attention to the "Beta Clutch Fix" and articles on water pump corrosion, which involves replacement of the inner clutch housing.

I had an 03 200.  After 11 years it needed new trigger coils and a bottom bush in the rear suspension.

Apart from that it was just routine wheel bearings, brake pads and chain/sprockets as far as replacements went.  One of the deciding factors, when coming from a Yamaha with grease nipples on the suspension, was a dealer review saying the back end could be taken apart in fifteen minutes.  It could but didn't need to be taken apart very often.