To comment on the K-Series processors for an investment needing to last 8 years, I would absolutely buy the K-Series CPU now. Your current Mobo does not support overclocking, but it also will likely not last you 8 years. I would expect it to need to be replaced in the next 3-4 years at which time you will still be able to find decent replacement boards, especially of the used variety. Also in 3-4 years when you are having to replace your motherboard, the best deals you are likely to find are current Z87 boards which do support overclocking. You would then be able to buy a used replacement board and overclock your CPU to give you an extra measure of performance to keep up with game needs. Also if you are the sort to sell used equipment, the Market for a used K-Series CPU will be double than of a non-K, so factor that in as well.
NOW, if you want to cut your upgrade schedule in half, then I would just go with whatever i5 fits your budget and do the same thing again in 4 years only then buying a nicer motherboard/CPU combo that will last longer. This might allow you to invest more money into a nicer video card which is probably your whole problem in the first place, but your question was about CPUs.
As for the H81 crippling an i5, I am an enthusiast, I overclock, so it is not a board I would use, but it will work and is hardly crippled. Just know going in there is no overclocking available and it will work fine.