I used to have a 156 V6 Manual and it's the second best car I've ever owned, only just being beat by my Current Italian Stallion,
The main things you need to look out for really are a recent cambelt change (Or budget to have one done if it's due) and as SA155 and Dan both said; The clutch. The clutch should be good for about 70,000 miles. Mine had 80K on the clock and had JUST had a clutch replacement.
So once again, opt for one that's either had a clutch done, or budget to get one yourself.
Another few things to make sure of are the following:
Ensure that both the red keys work the central locking, and that you have both red keys to begin with.
Ensure the car has the correct amount of oil in. (They DRINK the black stuff and if not looked after properly can starve)
Ensure that ALL electrics work, especially rear windows as these are a weak point.
Ensure that the panel by your right leg when sat in the car stays on alright, these often fall off and are *******s to get back on. Mine fell off all the time,
It's all little stuff really mate apart from the cambelt and clutch, so just look out for the usual.
In regards to performance it's like no other V6 I've ever owned, and I've owned a few others. 190BHP from a 2.5 V6 is pretty impressive when compared to BMW's 2.8i engine which hoists out the same power.
Being fairly light for a 4 door saloon is great because you'll see 60MPH in the extremely low 7's and a top speed of a touch below 145MPH. It really is a terrific and quick car, I'm not sure why I sold mine in the end, but I did, and now have something really impractical but quicker so we'll just leave it there,
You'll definitely need the 6 Speed manual, it's one of the best gearboxes around and will see off any other 156 V6 Variant I reckon. As long as you don't kid yourself that the 6th gear will return amazing MPG on the motorway as the best I got was about 26MPG, but then I never drove it like Miss Daisy was in the back,
Please, please, please though make sure you buy an SP 156 as opposed to a standard. The SP range all came with black speedo's with orange dials, and externally all came with side skirts and 16" alloy wheels. The main reason behind getting an SP (Sports Pack?) version is the uprated suspension. It's not amazing but still alot better than the standard high set up.
If you can, find one with leather interior and the big spoiler as this is, to me anyway, what a 156 should be.
The good thing is that 90% of people selling their 156 have no idea what specific model it is because it's even confusing for people who are aware of the SP range.
I think there's SP1, SP2 and SP3. You'll want (I believe!) an SP3 which comes with: Black Dials, Side-skirts, 16" Wheels, Leather Interior, Uprated Suspension, and the massive rear wing.
The good thing is, as mentioned above, no one hardly knows what the difference is, so they go for the same sort of money that the standard, boring ones go for.
For example, a 1998 156 V6 Manual SP3 with FSH and 59K on the clock recently went for £1900 on eBay, and it looked fantastic.
So yeah, shop around mate, find the best SP3 you can, and you'll be pleased as punch.