The IS200 doesn't have a MAF sensor as far as I'm aware, it has a MAP sensor. Yes, it's definitely worth putting in a K&N filter, it doesn't cost that much, lasts a lot longer than an OEM filter and can help mpg slightly. It probably very slightly helps bhp (1-2 bhp ?) but you won't notice that. I have had no problems with mine in the 8000 or so miles that it's been in and don't remember anyone else mentioning any problems either. If you do clean it and re-oil it yourself (there are kits for this), then follow the instructions because you need to leave it for a while for the oil to soak in, before you re-install it.
I'm about to put a fully synthetic 5W30 oil into my car in its impending service. The specs of oil are in the owners manual if you've got one, but there are some threads that Opie Oils posted (one of the traders on here) about selecting types of oil, try searching for their postings (e.g. this one and this one). It's worth checking out their website since they give discounts to LOC members. Different people may well have their own favourites for brands etc, personally I'm not sure the brand matters as long as the oil has a high spec (the various standards etc that it complies with).
As for spark plugs, the OEM spec Denso ones are SK20R11, and these are long-life ones. I replaced mine with Denso IK20R11 plugs, which are the high-performance variant. They don't last as long but then again they're cheaper too. Opie Oils seems to have some pretty good prices on those. Just as with the K&N, I don't think you'll notice your car going "faster" with IK20s in, but it can't hurt, and it might help your mpg slightly, who knows.