Servers

Only difference between the servers is when they started: Heaven's Tear, then Sanctuary, then Dreamweaver

Older the server, more higher level players it'll have. Also probably means more rare (e.g: legendary) items, highly refined items, that sort of thing ... TW (Territory War) map is going to be more 'fixed' with an older server (as in just a few guilds controlling most of the territory).

As for Cleric - IMO they're the second best solo class in the game - but only if you go for a more offensive build (priority goes to your attack skills, support skills are secondary). The expense depend's on how you play - if you're playing the support role a lot - which pretty much means doing anything in squads - then you'll be be doing a lot of healing ... which means burning through MP in a hurry ... which means using a lot of Spirit (MP) charms and/or pots. If you're playing more solo, it's entirely up to you whether you use charms/pots or just meditate. (To some extent, you can do both ... but there is of course an issue with having enough SP - or in later levels coin - to get both offensive AND support skills. But I wouldn't advise going purely offensive - as a Cleric, you should be able to serve in the support role, even if you don't want to play that way all the time.)

As for Veno pets - Golems are considered the best tank (out of the free pets). Other pets are better for other things though - Golems don't have a strong attack and are quite slow. And even as a tank, they're not the best against magic. They're also rather big, which can be a nuisance when your trying to click on mob. Bottom line is just because they're considered the best, it doesn't mean you need to use them. Another thing to keep in mind is it's expensive to upgrade your pet skills, but if you capture a higher level pet it'll have higher level skills. (i.e: If you plan on taking the Golem to higher levels, you'll need to upgrade their skills ... or you can just capture a higher level pet ... it may not be quite as good as the upgraded golem, but on the other hand, it's free ...)

As for pets vs taming ... first you attack a mob, get it down below half health (lower the better, just don't kill it!). Then you can use your Tame skill which (if successful) will give you a pet egg. Take that to the Pet manager and (assuming you have an empty slot in your pet bag) you can hatch it. At that point it becomes your pet.

Tick all the options and all the bars to the right.

EDIT: remember screenshots for advertising doesnt mean the actual game looks exactly the way you see in the screenshot.