3GS OS version 3.0.1 (7A400) and modem-firmware 04.26.08: how to upgrade to latest untethered OS

Hello, I have a iPhone3GS where I'm still running OS version 3.0.1 (7A400) and modem-firmware 04.26.08. I finally want to prepare the phone for my daugther with the latest possible untethered OS (which is 5.1.1 if I'm not wrong). I read a lot on iclarified, but still have some questions. Possibly it is easy for you to quickly answer these: - I used redsn0w_win_0.9.15b3 to try to save the SHSH blobs, but it failed. Is it possible that the old baseband does not use this feature? - do I have to use an old redsn0w version then? - also the trial to restore the IPSW (without saving SHSH blobs) did not work (iPhone2,1_5.1.1_9B206_Restore.ipsw), due to the fact that it tries to fetch the SHSH blobs any hint is very welcome. thanks a lot in advance, christian