I'm not an authority on Rodgers organs, but I do service organs, and have seen a bunch of these organs. It seems to me that you need to be brought up to speed on low end Rodgers organs.
When Roland bought Rodgers in 1988, from what I heard they were aghast at the state of technology and processes used at Rodgers. They worked like beavers for a couple of years transforming Rodgers technology into digital and also built low end organs in Japan. These models were the 440, 441, etc. With these I believe Rodgers lost a lot of credibility. Consoles looked and felt cheap. No more veneers for the exterior, they were vinyl clad. Early pedalboards fell apart in transit. Sound wise they were not great, but they did break a couple of cardinal selling points that Rodgers always used, that of being able to tune the instrument and voice them. These were plug and play instruments. Anyways, they did seems to work, and were no doubt good MIDI controllers.
Then in the mid 90s, Rodgers/Roland started building the 500 series organs in a Roland plant in Italy. If the early 4xx series organs were nothing special, the 5xx that then appeared were a definite step down. Everything was downright cheap about them. 520/530 were replaced with the 525/535, which were really not much of an improvement. In 2001 they revised the line again and came out with the 557 and 577. These were somewhat better. If I remember correctly, they also came out with a 527, which no doubt was a direct revision of the 525. These organs all used a single voice module, probably poached out of a Roland sound canvas, were not voiceable, used few and very short samples etc. In other words "extreme budget" comes to mind. In fact they didn't sound a whole lot better even with external speakers. The tone generator systems in all these organs would have been quite similar. In successive models they would have updated them a little bit and addressed certain problems or criticisms on these organs, so they did get better over time.
Rodgers realizing that there may be a problem, did a somewhat unusual thing, they produced versions of the 557 and 577 in the US, and called them Allegiant. They were badged as 657 and 677. These were built to the usual Rodgers standards, had upgraded audio system, and sounded better as a result. Also cost the customer $5,000 extra.
About the best that can be said for most of these Italian Rodgers was that they were good MIDI controllers.
As to comparison to Allen offerings, the Allen sound may have been uninspiring, but at least they didn't stoop that low in terms of basic build quality. Allen too has been offering cheaper organs, that they probably would rather not admit to building. In the 80s they used to advertise that only moving stop controls were any good, that lighted systems were garbage. Well in the 90s they came out with a lighted system, gave it a fancy name of "Lumitech", and pretend it is somehow all right now. Also, Allen now has a whole series of organs using Fatar keyboards, just like what everybody else uses. They used to always brag about the quality of their organs because of Allen-made keyboards.
I don't know how good any of these Rodgers organs are for running Hauptwerk, unless you just want to use keyboard action and expression. As far as I know, the Rodgers organs never did communicate well with Hauptwerk as far as stop action and pistons were concerned. I think it had to do with patch commands vs sys ex commands and how they were sent and received that were the problem.
You can probably pick up a 520/530 or 525/535 for very little these days. I wouldn't pay more than $1,000 for the smaller one, and maybe $1,500 for the larger one. Just remember, that low price was the reason these were made and sold. So figure that a short lifespan, plus no warranty, plus obsolete technology and sound, these would still be at the very low end of used organ prices. The other thing that has changed prices a lot is the fact that Johannus is offering new organs, full AGO for under $10,000.
You would be better off, if you can afford it, and find one to get an Allegiant.
Just remember, when the most important thing in a product is to make an organ at the absolute lowest price, like these Rodgers, there must of necessities be compromises, and usually that means little craftsmanship, artistry, cheap components, etc. The experience is never a grandly positive one.