Look for a used Lenovo T510, x220 or x201.
My tip is the T510 with i7. Or even x201 tablet version with touchscreen if you really need one.
Used Lenovos give best performance on low end price.
Above modells are all around 300-400€ so you will have some money left for the hybrid ssd like Andre suggested.

Only drawback i had to overcome is the wacky support of supplied Gobi2000 wwan adapter on linux, espacially ubuntu.