Ask for the "reimbursement" to be made in the form of a bonus so it's clear who owns the laptop - what you do with your bonus is your business. Yes, you'll have to pay taxes on it (I assume your tax laws count a bonus as income) but that's cheaper than the full price of the laptop and now it's yours, without question.
So long as there's 1) no contract to spell things out clearly and 2) the computer is "theirs", there will always be the risk of someone claiming the data on it is owned by them, not you. You may have an amazing relationship with your current boss but that situation may change (new boss, turn-for-the-worse in your relationship, whatever). If there's a contract in place, fine, but I suspect you're hesitant to ask that a contract be drawn up for just this situation.
Thus, if the company is willing to fork over the money, have them fork it over in a way that does not link it to the computer.
Or, of course, pay for it yourself. You obviously thought it was worth it given that you already bought it. Yes, an extra grand in your pocket is nice but you already decided you were willing to part with the money so if ownership of the data is important then you're not really out anything - it was an expense you were willing to pay.