You don't need the latest and greatest bike to be competitive in trials - many clubman class wins are achieved on bikes that are 20+ years old! At clubman level, trials is all about the skill of the rider and being "over-biked" is a common beginner mistake that will actually work against you when learning the fundamentals of the sport. If you only intend to ride for fun, a powerful bike can be "entertaining" but be warned that it'll easily get you into the kind of trouble that might result in unfortunate consequences! When competing, kids are restricted by machine type and engine capacity based on their date of birth - see the current ACU handbook and our trials information pages for detailed information.
It's true that there are no "bad" bikes on the market and it comes down to finding one that feels right for you. If possible, try as many different bikes as you can. Most manufacturers offer both 2-stroke and 4-stroke models although the difference in 2-stroke and 4-stroke engine characteristics isn't as pronounced as it is in the motocross or enduro world. As a general guideline, 2-strokes tend to be revvier and snappier while 4-strokes tend to be more "softer" and more tolerant of being in the wrong gear at the wrong time although they're more mechanically complex (and arguably more difficult and expensive to maintain). Popular beginner/novice level choices are a 200 or 250cc 2-stroke or 250cc 4-stroke.
If you're of smaller stature, you might also consider a 125cc or machine. Avoid anything above 250cc since these tend to be "expert" bikes and are oriented towards the kind of high-revving, explosive performance required at the higher levels of the sport. 200cc is a good compromise choice for a beginner. Talk to your local dealer - they'll provide an opinion about all of the current bikes, their respective strengths and weaknesses and (possibly most importantly), which ones are most expensive to repair and maintain but ultimately it comes down to your own personal preference.
It might be more difficult when buying used but try as many bikes as you can before starting to look for "your" bike. Avoid eBay and the like unless you know EXACTLY what you're buying because although there are honest bikes on there, there are also a lot of alleged "well maintained" and "hardly ever used" bikes that look sound but have actually spent the majority of their lives being ragged to death around a field by mechanically unsympathetic nutters and will require a lot of time and money to restore to serviceable condition. Solicit the assistance of someone who knows about bikes in general and ideally is a trials rider themselves since they'll know what to look for and when to walk away from a lemon. At a minimum you should check for:
Remember that you don't have to have the latest tackle to enjoy yourself or even be competitive if you intend to enter trials and it's all about finding a bike that suits you and will be your friend while you learn the basics, not a sworn enemy!