First let me say that my response may be biased, as I am a very satisfied Lexus SC owner. I have had my car for about 6 years and love it so much that I have actually sold it and bought it back. My car is a 1995 SC300 5-speed, but I have actually owned a 1992 SC400 as well. Needless to say I would choose the SC400, but I DO actually like P71s as well and have driven my friends P71 multiple times.
If you can manage to find a manual transmission sc300 I would personally pick that up over an sc400. Manual transmissions are a very important aspect of any car for myself personally, but manual SC300s are very rare. For example in 1995 they made 18,872 SCs total but only 793 5 speeds.
Price: Its incredible how cheaply you can pick up SC400s these days. I have seen clean examples for as low as a few grand. Considering they were 40K+ new that is a lot of car for the money. One thing to keep in mind while you are looking at SCs is that a "cheap" car could potentially end up costing you a lot more than a cleaner example that is a little more expensive.
This is due to some of the more common problems they have as they get older. A cheap example likely has bad ball joints(which are a part of the lower control arm). Replacement LCAs are $700 a side for Lexus parts, but you can run Supra LCAs and they go for about half as much. Cheaper examples are likely to also have bad bushings, burnt out clusters, cracked or split leather and split door panels(VERY common). Also look out for power steering leaks. Luckily for the door panels and other OEM parts there are a couple of vendors on club lexus that are constantly parting out SCs. You can get just about any part you need for a very reasonable price. The clusters can be redone by LexTech for pretty cheap. For bushings your best bet is to go poly. I recently put all poly bushings on my car, the kit was about $350 for everything.
Reliability: SC400s are extremely reliable cars. The 1UZ motors can easily run for 300K+. As with most cars I would look for complete maintenance records. I have put 80K on my SC300 while I have owned it and had no problems except for routine maintenance and things I broke racing. If you keep up on the maintenance the car will treat you very well! I put 15K on my SC400 in about a year and never had to fix anything on that car.
Power: SC400s have plenty of power for a nice smooth DD. They are by no means extremely fast but usually can run about a 15 flat in the 1/4 bone stock. 98 up cars have more power 290 hp vs 250 hp due to the VVTI, I also believe they have a 5 speed auto instead of a 4.
If you are looking for more power there are PLENTY of options! You can modify the 1UZ(www.lextreme.com), or you can swap in a 2JZ-GTE or 1JZ-GTE, or you can drop in a 4.7L 2UZ bottom end and use the 1UZ top end and ECU for plenty of usable power.
Looks: In my opinion the Lexus SC300/400s are one of the best looking cars to EVER come out of Japan, or any country for that matter.
Fuel Economy: Surprisingly good for a V8. The 1UZ was incredibly engineered and people get about 19/24. My SC400 actually got slightly better gas mileage than my SC300 5 speed.
Maintenance: Both the SC300 and SC400 are easy to work on and parts are easy to come by. I have never had problems walking into an autoparts store and getting exactly what I need! Parts are not super over priced either for the most part, I am sure the P71 would be cheaper to maintain however.
Fun to drive: The SC really shines in this area. There are tons of suspension and braking mods out there that can really make it even more fun to drive. Even when my car had only spring/shocks/tires it was a blast to drive and could keep up with just about anybody on the back roads! They are also really comfortable to drive. I have taken my SC300 from Portland to LA four times. I am 6'2" and 220 lbs and I fit like a charm, there is plenty of extra room for bigger drivers as well. The back seat is worthless, although I have fit 3 girls back there several times haha.
As far as the auto tranny issue you can swap in a manual tranny. With some modification you can run a getrag 6 speed, w58 or R154! If you do a search on Club Lexus you can find some threads with more info on the manual tranny swap, there also is some info on lextreme. An SC400 with a manual would be a ton of fun, I wish they have produced cars with those options in the first place!
As far as forums the biggest one by far is Club Lexus (www.clublexus.com). There is also a forum on lextreme. I spend a lot of time on Supra Forums because the two cars are so similar. Another good forum is an Australian forum that I prefer to Club Lexus because people there actually race their SCs/Soarers, www.soarercentral.com. You can also check out www.clubna-t.com for info on going turbo.
As was already mentioned the Lexus SC has a chassis that is extremely similar to the MKIV supra. The supra was built off of the SC/Soarer chassis, the supra is just like 3-4 inches shorter. Many parts swap over form the supra. Subframes bolt right up, most suspension pieces, brakes, drivetrain, seats, etc. I have many Supra parts on my SC.
If I were you I would buy an SC400 and never look back. If you do buy one, I am sure you will love the car! I originally bought my car as a daily driver and had an NX2000 as a race car. My SC has actually taken the role of fun car and I purchased a different DD. If you have any more questions I would be happy to help you out!
Josh