Buy a msport 530i and swap the manual bits into it. Not quite as fast as a 540i but way more reliable, better handling and you can use most of the parts from your 528i (540i mostly limited to interior, exterior and rear suspension parts from 528i). The average auto 540i is too much of a ticking timing bomb with timing chain guides, unreliable transmissions as well as the usual E39 issues. So unless its been owned by someone like @hqstu or someone who has thrown lots money at it, id probably avoid it.