Lexus is the 'luxury' division of Toyota (with all that it implies to costing)
While the dealers get a well deserved reputation for customer service - my car was subject to a recall a couple of weeks ago and after the remedial work was done it received a full valet, £25 worth of fuel and I received two bottles of quite a decent Spanish wine for the inconvenience - that reputation comes at a price.
During the additional but complimentary 'safety inspection' the following was found - lifted from the report - "Right hand rear calliper bracket seized, requires bracket pins and boots, requires handbrake cables both sides - sticking". The cost? - £1080.59 of Uncle Denny's spondulicks.
Although many of the parts are interchangeable between the two brands, as long as you can get a friendly Toyota dealer on your side you pay less for the same part by buying through them but this can be difficult as they are told (from my investigations to date) not to do this.
Insofar as the IS is concerned, I've read that some people consider them to be thirsty and quite firm in ride so it may not suit your purposes especially when the cost of parts from Lexus and the eye-watering service costs are taken into consideration. They are also quite snug inside.