Yea for your size the X would be a better fit , especially for the long run as far as size goes , as far as "maintenance" goes , standard oil change , lubing , greasing bearings , normal dirt bike stuff .... for the valves , what most of us do is when the time comes and the valves zero out , then the best bet is to swap to Stainless , its basically a 1 time cost and you can ride without worrying about them again , for the rest of the time you ride .... example: , I put stainless intakes in my 06 450X in 06 ... I have not had to do a valve adjustment since .... though the valves do wear (the stock Titanium) what mainly kills them is dirt getting past the air filter ...(part of regular maintenance) so keeping a clean well oiled air filter is critical to extended valve life ....
If the old owner has any maintenance records , it would be beneficial for you , so you know what you are getting into , as far as what has been done in the past and if there was anything extensive done .... my 06 450X (results may vary) is still on the original piston , clutch , etc, , the only thing that was "needed" was the new intake valves , and that was my ignorance for not keeping a eye on the airbox to see if dirt got past the filter , well it did , lots of it , so my valves went , lesson learned ... but other than that and adding a HotCams Stage 1 for trail riding (all bottom end and mid power delivery ..... my choice)
I have basically only done "regular" maintenance , which as mentioned is seasonal bearing repacking , for swingarm linkage , swingarm pivot point , wheel bearings , and steering head bearings .... oil changes depending on conditions , the dustier the more frequent the changes , just like any other vehicle , same goes for the air filter .... if you don't rev the crap out of it , and keep it relatively maintained , I do not see any reason why you can't get a decade out of that bike without a major malfunction ... I am at 15 years and counting , on my 06 450X on original parts , including the brake and clutch levers .... My 02 450R has over 400 hours on the current piston 400 hours , think about how much actual riding that is ... and that is track/mx riding , it is also still on the original clutch , I have had that bike since 08 and paid $1,650 for it , the only major things was a piston , just because I felt it was time (this was over 400 hours ago mind you) and the intake valves just like my X , same scenario , dirt past the filter (this was not me as much as a issue with the 02 specifically , changing the airbox/filter to a 03 solved the issue along with the new valves)
What I am getting at is any bike can be a maintenance nightmare , as well as they can be almost trouble free ... there is no guarantee , but if you do what the Owners Manual says (minus the 20 hours for a piston stuff) and just keep up on stuff and don't abuse the bike , there is no reason you won't enjoy that bike for a long time to come ...... the same goes for most bikes out there , whatever the brand ....
One other thing .... basically when buying a used bike , do the same as you would for a used car , maintenance records are helpful , as is looking the bike over closely for excessive wear or abuse ..... the clutch cover is a good indicator , if the owner says it only has a few hours on it and the coating is completely worn off the cover , chances are its been ridden more than claimed , if the bike is on a stand (not the kickstand but a lift stand) grab the rear wheel and try to move it side to side , or up/down , if it slops up/down , that is a indication it needs shock or linkage bearings , or both .... look for leaky fork seals , just another maintenance item , but the more you need to do at the beginning , the more the bike is costing you initially , replacing them later down the road is part of maintenance , but needing it just to ride it , means you just paid more for the bike then you expected , so you need to consider that , and if the list of bad is too high , maybe keep looking for a better deal ....
