(1) The difference is big. 1.5 GHZ Celeron-M is about 2.0GHz P4, so P4 will be 50% faster. However this is only matter if you want to do video editing/encode (HDD too small for this anyway) or play latest game (integrated graphics is not good for this anyway).
(2) Replace RAM, I would say easier than desktop.
(3) battery life? All depend on what you want, if you want a desktop replacement, short battery does not matter, if you want to move around, P4 lappy is not good.
A few points:
(a) P4 lappy is hot and noisy
(b) a few alternative for you
HP Compaq nx6110 Intel Celeron M 360 1.4Ghz / 15 XGA TFT / 256MB SDRAM / 40GB HDD / Combo / XP Home / Notebook PC £455 click here
FUJITSU SIEMENS Amilo Pro V2010 C1.5Ghz 256MB 40GB DVD/CDRW £448 click here
ACER ASPIRE 3002LMi AMD Sempron 2800+ 512MB (2X256) 80GB DVDRW Dual XGA 15TFT XPH £519 click here click here (cheaper model in comet click here)
HP 3000+ Power Model offerd in Tiny (£548) click here