Got a nice setup dude, should work good for you. As far as which unit I'd read reviews, theres alot of pieces of sh*t out there, even some with big price tags. I'd also buy it from a big retail store if possible and get very clear information on their returns/exchange policy just in case it turns out to be crap (I would count on it sucking, and if it works hey bonus). If it worked as advertised yeah thats nice. Sounds like you need to cool at least 20 degrees in there and you'll probably lose some efficiency on your hot water tank but I suspect thats something you'll just have to deal with. Basically I think you want something huge to cool that much, but then again remember we are in the middle of summer and most summer crops suffer because cooling costs are ridiculous in terms of both electricity used and cost of decent units to cool them.
I'd probably look at 10-12k/BTU models make sure the reviews dont say anything about them leaking. I'd plant one in a rubbermaid container just to be safe, I've heard all kinds of horror stories about them leaking. But sounds like you got a sweet spot to vent hot air, right up and out of the grow room, so if you get a good strong unit that isnt a piece of junk you should be g2g. How much it will cool your grow room I dont know. I know for a fact HD where I live has a great return policy, you could pretty much buy one, use it for a week, check your temps and return it for whatever reason you want if its not big enough. I was actually just looking at an 8k BTU unit there today in fact. You could always buy one, start small, if thats not enough, return get a bigger one etc.
I wouldn't order something like that online, return process would be a pain in the ass if you had to return it, and I've heard of tons of people having issues with portable ACs, either not cooling properly, only fans will blow but no cooling, or they leak, etc.
Also I think it would help to insulate your hot water tank so not as much heat escapes and radiates through the room.