Writing with a fountain pen is a totally different experience (as compared to writing with a ball pen or roller).
You can feel that even when you try out the low-priced Lamy Safari.
People who like writing and the good/quality paper + fountain pen experience find endless satisfaction even with a low-end pen.
Does she like calligraphy? would she enjoy thin and broad nibs at the same time? or will she use it in the 'office' or in more formal situations?
If this pen is to be used privately, i.e. at home, or if she is a freelancer, it could be a Lamy Al-Star or Safari Limited Edition - then you could buy her a couple of nibs (fine/med/broad/italics/oblique at 4 quid each) so that she can change them, play a bit, see what she likes, feel the real difference in using and (if you like) messing with an fp, changing ink, etc.
Alternatively, you can go and buy her a Lamy Studio, which I suppose is similar to the one you had you eyed originally.
The Heroes, low-end Watermans and Pelicans I really don't know them, and have no clue on how much they cost.
A vintage is always a very good and admittedly a much better choice, but I cannot really comment on prices, etc. as I don't know the market and also do not know if you would be willing to buy smt like that for your sister.
All in all, try to find out more from her, if you can. Prep (remember shaving) is as important as execution. The person you are presenting the gift to will always remember the gesture for this reason as well. Then buy something she can also change if she would like something different (but she won't if you manage to get some 'inside' info in advance). You can also try Andy's pens <!-- m --><a class="postlink" href="http://www.andys-pens.ukhome.net/forsale.shtml" onclick="window.open(this.href);return false;">http://www.andys-pens.ukhome.net/forsale.shtml</a><!-- m --> or give him a call, he should be able to help.
Another site here where you can see different colours of ink, pens and nib sizes.
<!-- m --><a class="postlink" href="http://www.flickr.com/photos/biffybeans/show/with/3821895190/" onclick="window.open(this.href);return false;">http://www.flickr.com/photos/biffybeans ... 821895190/</a><!-- m -->
Good luck and let us know what you got for her.