the wheels will 'fit' your cars stud pattern but like you say they wont go over the calipers(front). and those tyres are way to small for a 400 so you'll need at least another £250 for a cheap set of suitable tyres!
as they are 6.5j with a 50offset which is the same i think as my 97 model, so by the time you have spacers they'll stick out a bit. you can't get a simple slip on 15mm spacer, you'll need a 20mm bolt on spacer(about £100 if you can still get them, but then the centre bore is wrong for a lexus normally so you'll have to get them machined), but if its anything like mine your studs will be about 25mm long so 20mm spacers are no good as the wheel will not sit flush against the spacer, just against the stud ends. so you'll need to fit 5-10mm spacers first before the 20mm bolt ons!........you still following?
i've got mazda rx8 wheels on mine which are 7.5j with 40mm offset(i may have got the offsets muddled up).it looks good but what a carry on! i now know why you dont often see a 400 with nice aftermarket/different wheels on!
personally i re-sell the wheels and try and find a 400 specific set in 17" or 18" as standard size 16" is too small......just to note you can go bigger/smaller with the offset(stick out more) as the standard wheels have plenty of space inside the arch, think i've got a photo of my fronts with 7.5 wide wheels with 50 offset with 30mm spacers! think they stick out about 5mm, illegal but not really noticable(passed its mot)
sorry