If you have decided to invest in a brand new instrument from a retailer, the considerations are quite different to those relevant to buying used. We can immediately rule out instrument faults and imperfections, because new purchases are covered by guarantee. So, I'm going to talk about which brands, models and retailers I would consider.
If you already have a good relationship with an independent music store in your town, then of course - go and talk with them about your requirements, but as high street stores are getting rarer, I am going to recommend GEAR4MUSIC online (click the text for link). They stock a decent range of instruments across all departments, and their service and advice is spot on.
For advice about brands and modes... keep reading:
My top recommendation is for YAMAHA instruments - having owned the same yamaha trumpet for 14 years now, it has been played hard, and I'm ashamed to say, looked after badly .. it is a pleasure to play, consistent, expressive, versatile and extemely well built. I play the Xeno 8335rgs (now discontinued but replaced by updated models).
A word of controversial warning from me though ... The student model trumpet YTR2330 - is made in China rather than Japan, in line with Yamaha's policy with their lower end instruments. Now, I am reading online that there is nothing to fear from these Chinese instruments, but my personal experience tells me otherwise: Working in schools, I have encountered 3 trumpets of this model, where the little finger hook snapped right off... and that is not something I see as a common fault with small brass at all. Depsite this, the instruments play nicely ... I would just be wary of the durability and strength.
The rest of the range is fabulous! Spend as much as you can afford because it is a great investment. The extra money goes towards better metals (which effect the sound quality and the feel when they are played) and the precision of construction (which improves tuning / pitching / accuracy and reliablilty of valves and tuning slides etc)