This is asked quite often by those wanting to get involved in astrophotography at a far lower price point and there is nothing wrong with getting started on a limited budget and making compromises. That being said, I think you are being very conservative in estimating costs.
As others have pointed out, the cost of a set of narrowband filters will be around $1200. You are already talking about replacing the focuser and that will be at least a couple hundred more in cost. And, a field flattener would be a good idea. All these upgrades are adding to your total startup cost.
Back to the narrowband filter set... some beginners are not bothered by the chromatic aberration. Actually, most non-photographers wouldn't even think it doesn't belong. That is a personal preference though.
The cost of a used AR152 typically, in today's market, has been around $750+, not the $500 you are estimating (unless you already have a seller lined up, of course... then that would be a great buy if it is in good condition!). Incidentally, I thought the AR152 had a rack and pinion focuser but I could be wrong and, if buying used, it could be any focuser on the scope and not necessarily the original stock focuser.
Using your CEM26 for imaging with this telescope, guide scope, camera, and whatever else you add into the mix, will be an exercise in frustration. As with any part of this budget exercise, it can be done but many compromises will need to be made and you should not expect consistent results. The weight of this configuration is one problem because you will be hovering around the maximum capacity of this mount but the long moment arm of this big scope will actually be a bigger problem. For observing only, you could probably get by if you are more tolerant of waiting on the scope to settle. For imaging, it will be an ongoing exercise in frustration. I would think that a CEM26 might be the biggest problem in this plan.
My opinion is that there is nothing wrong with starting down the road to astrophotography with an achromatic refractor as long as you understand your limitations, the compromises, and you are realistic about your expectations in the resulting images. Again, unfortunately, you might need a heftier mount though.