Catalytic Converter Quandry

[Philoprof]

The right catalytic converter of my 380sl has been rattling, particularly when I start the car on cold mornings, and the sound dissipates shortly afterwards. I had a custom mufffler shop put the cat on for me -- a universal Walker, if I recall -- about 5 years ago, because the straight pipe system they made for me was just too loud. So, replacing the cat at this point should be fairly easy.

I suspect that the car's burning oil because of leaky valve stem seals on 3 cylinders contributed to the cat failing. But I recently read that a too rich mixture, or using gas with too high an octane rating can also burn a converter out. In fact, I've been using premium (93) for a few years now, because I was getting a significant amount of pinging with regular, even though I think MB recommends regular (89) for the car. I was therefore wondering whether anyone else here has had any problems using premium in a 380sl, another 107, or any MB requiring regular.

In any case, as I'm getting ready to replace the passenger's side converter with a Magnaflow universal, I was wondering whether I should replace the cat on the driver's side with a true cat, a pre-cat, or simply a pipe, considering that reliable sources have told me that modern cats don't require pre-cats, except on 3-way systems having dual O2 sensors.

In fact, while researching exhaust systems for the 380sl, I noticed
that some have driver's side cat's, some have pre-cats, and others seem to
have just pipes, at prices that differ significantly. For instance,
catalyticconverterwarehouse.com has a driver's side cat made by
Quali-cat for $67.61 delivered; convertergeek.com has a DEC that has what appears to be a pre-cat for $93.06; and there's a whole system from Eastern for $311 (40061) that only has pipes on the driver's side.

I'd obviously prefer to go with the Quali-cat for 67.61 on the driver's
side, since money is an issue right now. However, I've never heard of Quali-cat, and googling it didn't turn up anything on the company. So any advice on the best way to go from people who may have researched this matter would be greatly appreciated.

Thanks very much in advance.

Bill