So in summary,
Nothing wrong with the Juwel 450 as supplied, 1000litre/hour built in filter, lights well and truly in the low & plant safer end of light intensity (PAR). A great safe start for fish and plant keeping.
I ran my Vision 180 for years with the internal 600l/hr filter & 2 x T8 tubes. Didn't have much success with plants (plastic plants did appear to do OK though
🙄) as I didn't know what I know now. One issue with the internal filters, possibly as they are not that powerful for tank size and also stuck in one corner, is that I used to get quite large detritus (fish poo !!!) deposits in certain places in the tank, as there was obviously no flow in those areas. An air pump with "bubble wall" across the back of the tank helped considerably, obviously preventing the detritus settling and allowing it to be filtered away. I also tried making a spray bar across the back of the tank, connected to the Juwel internal filter outlet, prototyped out of PVC pipe, but the Juwel filter pump was clearly not powerful enough to produce any decent flow/circulation.
Again look at Zeus's tank for how to do high tech in 500litres. Not simple and cheap, but I feel he managed it.
https://www.ukaps.org/forum/threads/olympus-is-calling.43046/
You next step up would be better filtration, problem is with such a big tank it is not easy, especially as one of the tank "inlet cutouts" is blocked by the internal filter, if you keep the internal filter. If going high tech you are looking at, at least 4500litres/hour filtration rate, not cheap and not easy to do within the confines of the Rio 450. Even two times JBL1900 giving 3800 litre/hour is not really enough, plus you will have all the faffing around fitting 16/22mm CO2 injectors etc in 19/27mm piping. Two times Fluval FX4 would do it (2650l/hr), again watch the pipe sizes, you would have to make your own spray bars from PVC pipe as well (see Zeus). The FX4 is huge, check they will fit in the cabinet.
Lighting is the next issue, especially if you want to keep the Rio450 hood. Using Juwel 150cm T5 fluorescent units, with reflectors, will double your light output. There even might be an iQuatics 4 tube unit that fits the Rio 450 giving 4 times. Beyond that there is the Aquatlantis T5 LED units and beyond that you are looking at DIY'ing more powerful lights into the hood (Kessils ?).
You need also to think of maintenance, can I easily reach down 65cm ?, how am I going to add 225litres of dechlorinated (warmed ?) water ? Does not Zeus have a mixer shower valve located near his tank ? What are you going to do with the waste water ? I water my front lawn with mine.
So lots to think about...in moving a big tank beyond what the manufacturer supplies.