firstly - leasehold for houses is actually more common that you might think. many do buy up the leasehold at some point to become freehold, but in some cases (like mine) there's no point - with a 999 year lease and a low ground rent (mine's £15/year payable in two instalments of £7.50!) it's just not worth buying the leasehold out. you also have rights to extend the leasehold as well, once you've lived there for two years - worth reading up on those rights in advance, just so you're aware of them. most new builds are also leasehold, too, these days. part of the reasoning is that leasehold can impose conditions to keep the neighbourhood at a certain standard and in a good state of repair, for example - a rule about no flags, or fence heights, or paint colours etc can be enforced via leasehold conditions.
in my case, the ground rent is fixed and has no provision for increase, which means that they can't suddenly up the ground rent. in the case of many new builds, that provision isn't in place - so what starts as a reasonably-priced rent gets escalated, and that's a major issue. after all, if the rent gets escalated too high, no-one will buy it, and you're basically trapped because you can't sell on.
in terms of marketability, leasehold does put some people off, yes. but if it's very long term and very low rent, then for most people it won't be an issue. if yours is good title rather than absolute, though, that could make a difference. ultimately, you're considering it so you have to assume that someone else will also consider it in the future as well in the same way that you will. a short leasehold term is usually more problematic - i've had friends in london, for example, with lower than 50 year leaseholds which are a much tougher sell.
if you can, find out why it's good title rather than absolute, and whether you can upgrade it and/or buy it out. usually it's because the registry has no information about the land prior to the granting of the leasehold title rather than anything to do with the leaseholder themselves - presumably, they still exist and you're still going to pay them ground rent and ask permission from them etc. indemnity insurance is usually used in these cases just in case someone does turn up with proof that the land was theirs before the leasehold was granted. in practical terms, it's pretty much the same as an absolute title, though.
in terms of extensions and alterations, i can be a bit more useful, though. turns out that those clauses are time-limited. in my case, the company who own the leasehold still exists, but the previous owner extended twice without asking permission. legally, if extensions/alterations were more than 20 years ago (in my case, they were made in the 70s, they're beyond that limit), the freeholder is legally time-barred from taking any action. so, realistically, if you're staying there for the long haul it shouldn't matter overly. usually, the leaseholder doesn't generally care all that much - for £15/year, it's not worth their time, effort or money to check or to enforce it. in my case, i should ask for consent from the leaseholder, and will likely have to pay them a fee to obtain that permission.
imo, it's not an unusual situation, and you don't really have anything particular to worry about other than the fact that the existing owner didn't ask for permission. usually, when the leasehold is transferred, the owner will ask whether that was done - if it was in the last 20 years, then that's a potential hiccup and the owner will need to clarify and sort that out or obtain retrospective permission for it. ultimately, though, that's their issue - worst case for you is that sale falls through. more likely, the seller will pay a fee to resolve it and/or obtain indemnity insurance against there being an issue. that basically covers you for any future issues with the leaseholder as a result of it.
if in doubt, ask your solicitor about the legal side of all of this, though - that's what you're paying them for!