I am an international architectural photographer and I happen to own a P45+, P65+, Aptus 12 and IQ180, so I will try my best to answer you questions, based on lots of real-world experience.
I really like how the Leaf/Mamiya backs generally render architectural images. The profile is a little gentler than Phase backs render things. Generally, the highlight transitions are gentler and shadows are more open. This is what makes these back so awesome for portraiture.In architecture, this is particularly valuable with daytime exterior shots if you shoot in a location which has high-contrast daylight (most of Australia, California, Spain all come to mind). This generally holds true across the Leaf range, though my recent experience is confined to Aptus 65 and AptusII 12 backs.
However, if you are shooting into twilight and beyond, then the 30second limit on long exposures can be a real handbrake. I find that in order to be shooting at my lenses optimal apertures (f8 -f11) I am more often than not in the 45-110 sec range.
The P65+ is so far, my favourite back for architecture, though I think the IQ180 will take its place once we have a better LCC solution. It has a 1 minute "limit" on long exposures, which in practice, I very rarely hit. I have shot with it up to 90sec but with the use of frozen gel packs to keep the back cool. It's a bit of a pain, but it works. The advantage this back has over the P45+ is the slightly larger sensor area makes all my lenses just a little wider. When shooting interiors, you will need one lens in your kit that is seriously wide. [Yesterday, I was shooting a bathroom at the soon to be completed Grand Hyatt in Abu Dhabi with a Rodenstock 23HR on the Alpa and IQ180 and I was wishing I had a 21]. The P65+, because its sensor is larger and has smaller pixels than the P45+, exhibits greater lenscast colourshift than the P45+. However, as it remains within the correction range of the software LCC solution in C1, in the realworld, it is a non-issue. You have to correct every shot with the P45+ if using tech-cam wides anyway. Correct use of LCC gives you great files with both backs.
This brings me to the P45+. If, as you suggest, you are not yet working at the high revenue end of the market, then I think this represents value that's hard to beat. It's well proven, is capable of longer exposures than you are likely to ever need (1 Hour if memory serves me) and is not a massive crop on the full-frame sensors. At current refurb prices, this is the back that I think offers the best ROI. I have kept my P45+ as my backup and also for that once-a-year shot that really needs long exposures.
Note that I am not saying the P45+ is better than the P65+ for architecture - I am not. As you have indicated that cost is an issue, I think the better strategy from a business standpoint is to get the P45+ and make it earn your way to whatever your next back might be.
Don't let anybody tell you there is no money in architecture photography. Pick your niche carefully, do great work and nurture your reputation and you can do pretty well.
Best of luck.
Cheers,