As an owner of CPO Bimmer, I'd also suggest you to look into it.
Also, keep in mind that, when you return the leased 3 series, you should have OEM Runflat tire with certain tread left on (unless the term specifies otherwise). Most people do either 1) Take out OEM tire (OEM RFT sux and only lasts like 15k miles anyway), get non-RFT replacement tire + temp tire, drive and return with OEM tire back on, or 2) Drive as usual, replace it as usual, and return with new/close-to-new RFT.
Option #1 is a cheaper, which would cost you about $ 800+, and let you free from crappy RFT ride, but will be hassle as you should replace it several times and have to carry temp tire with nowhere to properly store.
Option #2 would definately cost more, at least $ 1500 as RFT's are expensive and you may have to replace it more than once, but the cost is pretty much only concern if you can live with crappy ride comfort.