so, if I understand correctly.
you have a semi circle shaped room, with a domed ceiling? (I assume that the domed ceiling is highest near the pulpit and slopes towards the back?
except, it's not a semi circle, it's a bit smaller, I'm guessing it's narrower than a semi circle, but wider than say a shot put pit?
what sort of square footage is the floor? it the ceilings are 40ft high then the floor must be a reasonable size...
in which case... here comes the bad news.
your echos are unavoidable, the room is bad for sound, the best that you could do is try to invest some serious time in deadening the room. that would be with the use of heavy curtains, wall tapestries, anything that will absorb sound really. if there is no carpet, then think about whether that's a possibility.
I imagine that while not an old building, the building would be built to be echo-y, that's how many churches were designed, the pulpit is at the front, the acoustic properties of the room help sound to travel to the back, with reverberation. such that someone with a loud voice will be able to be heard without amplification...
The second bad news is that no amount of wattage is going to get rid of your room problems, in fact it might just make things worse. and to be honest you can bathe a busy pub in sound from a 100Watt mixer-amp portable PA system so your amplifiers and speakers are ridiculously over powered for this application, you can probably spend less than half on each of these items... if you're going with this variety of PA, look for 100 - 250 Watt speakers. your saving in money could be used to create a larger system with a second amplifier or more speakers at different points, which would allow your system overall to be run more quietly. (speakers closer to ears means speakers not having to run as loudly).
Yamaha do make better PA systems for the purpose you want now than Peavey.
Though I'm sort of tempted to say don't look for band/pub/club systems at all.
For a church venue you'll normally stay away from the kind of equipment that you're looking at completely.
rather than looking at 8ohm, 2 way speakers, start looking for speakers purpose build for what you're doing. you'll find a professional installer would recommend 100v line speakers, possibly an array of them, positioned high on the wall and pointed to direct sound downwards such that the echo was lessened.
What are you planning to put through the amplifier? just voice, or do bands play there? will music go through the system. (I mean music for people to listen to during a service, not background music that may be played when you're moving into or out of the church.)
Also will this be a permanent installation? -i.e are you planning on bolting the speaker to the walls? -do you have any idea how heavy the speaker cabinets that you linked to are? the bass drivers are 15"! those are huge. do you really want to end up with the speakers on the floor because they are simply too heavy to mount effectively to the walls? also, if it's permanent do you want to look at speakers with a more binding post style mounting such that speaker wire can be permanently installed...
in your situation...
I'd be making the following choices.
Desk, you made a good choice, I think a 16channel USB desk is a little bit overkill, but it'll enable you to expand in the future, make recordings of sermons etc to PC. the non-sale price is $550, which i think is a bit much to spend on a desk that you so far apparently only plan to run a single mic channel though. but if you can get it at the sale price of $299, (which it says that you can't on the site). then get it...
Given the room that you have to work with, (and the perchance for feedback/echos etc). I'd also get one of these DBX DBX231 Dual 31 Band Graphic Equaliser
it'll give you finer control over problem frequencies than the desk will.
After that I'd probably go to a decent 4 channel home stereo amplifier. (so not a PA amp at all).
I won't link to a hifi amplifier because I have no real recommendation, just get one that's got four speaker outputs, (so two left and two right) for the most versatility.
I'd have 2 floor standing speakers at the front,
floor standing speaker - Google Search
and 2 "bookshelf" type speakers at the back fixed high on the wall and pointing at the floor in the centre of the room.
Bookshelf speaker - Google Search
my second choice would be the same desk and eq, but with a line voltage amplifier/transformer TOA A 2120 120 Watt 100V Line Amplifier
set-up with six or eight line speakers placed evenly around the room, (wall mounted and high pointing down towards the centre of the floor.)
line speaker - Google Search
As a last resort, I'd go with what you're planning on doing now, and get a pub/club PA system that's meant to compete with hundreds of people drinking and shouting, whilst also deafening them and put that into a church... with only two speakers, that are too large for wall mounting and therefore have to be sub-optimally placed.
(when I describe the system like that do you see why I say that a "band" PA isn't a good fit for a church? and would be my last resort?)