I open machete and 5 pots and rush a BotRK. After that I get boots, Ninja Tabi 90% of the time, and then i'll rush my Spirit Visage.
After that, it's completely situational. BotRK and Visage are an absolute must. With just those 2 items, you become virtually unkillable, and there are VERY few champs that can straight 1v1 you. BotRK's active combined with Trundle's R is extremely brutal and gives him ridiculous sustain during teamfights. Add a visage to that and forget about getting killed.
Once I grab those two, I usually go for Hydra as my other damage item, then an IBG. I rarely get a 6th item with Trundle, but depending on the enemy comp, you can grab something like Thornmail, Sunfire, Veil, Warmog's, etc. It's strictly what you need, but it should be a defense item since BotRK and Hydra are more than enough for your damage.
If you prefer, you CAN grab Spirit Golem over IBG, but I find myself selling my Machete in favor of an earlier Visage.
Top lane Trundle is also very good and you can use the exact same build, but i prefer jungling with Trundle since he counter jungles very well and is hard to counter jungle himself. --- Best thing in life: Sex Second best thing in life: Pizza