no point in swapping for a motor/car that already exists. you'll spend more doing a swap unless you happen to come across a wrecked M for cheap and plan on doing the work yourself. if you're bored, then i say go big or go home. go for a v10 swap from the m5. the motors usually can be had for 10k or less on ebay.
TC Kline | Ground Control | MFactory | Recaro | Supersprint | Active Autowerke | Volk
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