Rhuester, if your friend builds these ponds for a living and wants to help you build one for the price of a few beers plus parts, I say go for it and have fun with it. If you are interested in growing out some koi to be big, or seeing this end of the hobby, look around for a local koi club and find out when their next show is and go to that. You also could go to a club meeting and just let them know that you want to build a pond and could you possibly look at some of their ponds and koi. It might be an eye opener for you.
It would be great to jump into this hobby right from the get go with a purpose built koi pond. most of us haven't and I suspect if most of us would of known how much money we would of spent on this hobby we would of run away as fast as our legs would take us.... we would of then missed out as well on the joys of koi keeping and the friend ships that only fanatics that share a common obsession can have. Kind of like the love of one drunk for another.
it's not the usual way into this hobby to jump right in with a proper koi pond, some of us do, but that is pretty rare. So start with your water feature, and some gold fish with your bog and see how you like it. I actually wish I had the space for something like that, not to replace my koi ponds but as a separate thing. The clean outs might be a lot of work and hydrogen sulphide stink isn't a pleasant thing but the trickling sounds of a little stream and the flowers on a lilly pad and some bright orange little fish darting around might just make it all worth it.
To sum up, I'm just saying I don't want to scare you away from the hobby and I'm sure from all the posts on here you can see there are some strong feelings about aquascape ponds full of rocks but we all start somewhere and it's hard for us to remember that.
happy ponding