He didn't expect to inherit Delaford when he went looking for her, I don't think. Colonel Brandon's brother died and left him Delaford 5 years before the action of the story. Eliza died 14 years ago. So Colonel Brandon would not likely have been expecting to inherit Delaford 9 years before he actually did.

We don't know how much older his brother was, but presumably he would have only been in his 30s or 40s when he died. We also don't know the cause of his death--possibly due to those 'pleasures that were not what they ought to have been'? At the time Colonel Brandon went looking for Eliza, he did go and ask his brother for some clues, and there was no indication then that the brother expected his death was imminent.