Well Jeff, this is a link to a (German) wheel dimension converter. Put in the original size, then the new one, and it figures out exactly
- rolling circumfence; difference thereof
- track / tread
- chassis clearance
- deviation of speedometer reading
I am sure there are similar converters on the net that speak English.
As for the rim - just browse for a tire configurator, such as this one (German again; go for Lexus - LS - F2; choose from 50 different 19" rims; or even more 18"s). If you stay within the prescribed offset you won't have to do any extra work on the body. Standard tire size for 19" rims is 245/40.
I have 18" rims on my '98 LS, and I'd say that's way enough. It does impair ride comfort! Standard tire size for 18" rims is 245/45. 19" rims aren't so much more expensive than 18", it's rather the rubber that runs down your budget over the years.
Click here for some pictures of my car with 18" TTS GrandStand rims (original suspension, no lowering).
Good luck, good taste, and good sense.