It depends entirely on what it's worth to others, worth to you and what an alternative would cost with the same or better functionality. Worth to others, in current condition, can be found on kbb.com and local listings. Alternative vehicle costs can only be defined when you detail your specific needs and wants. Worth to you can only be defined by you.
That said, literally every vehicle needs maintenance/upkeep. There are 3 main areas which are generally considered life-of-the-vehicle items on the Sienna... Chassis, engine and transmission. If those are good, you can expect a long, long life out of your van. FWD versions can easily last 300k miles and AWD versions typically last at least 200k miles. From that perspective, all the suspension components are normal wear items, which you would expect to replace every 100k miles or so. You would also want to do the timing belt/water pump/seals and probably the spark plugs, rear ignition coils, valve cover and throttle body gaskets. Lastly, check the condition of the vacuum lines (which tend to degrade around 15 years) and consider replacement of all of them, if any are degraded.
So, you've owned the van since new. What is the maintenance record like? Done all the regular stuff (oil changes, alignments, tires, etc.) at appropriate intervals? Any signs of significant chassis/body rust in critical areas (i.e. rocker panel, rear subframe, etc.)? Any other significant deficiencies which might make it unsalable to a potential buyer? With your low-mileage mini-van, assuming no other significant issues, you could expect to get somewhere around $5k (maybe less; probably not more) for it. Could you get an alternative vehicle which meets your needs for that kind of money in your area? How is the cooling system? Any leaks or need to add coolant ever or ever have any overheating? And the transmission/fluid (fluid color, drain and fills and/or filter change done, any wetness around where the fittings enter the radiator)?
In the end, it's probably going to be a personal/subjective choice. Doing the whole front suspension will probably be $1200 in parts and another $1000 in labor. Doing the timing belt will probably be $150 in parts and $600 in labor. Spark plugs, coils and gaskets are probably $400 in parts and another $600 in labor. Dropping the pan and changing the filter will probably be another $100 in parts/fluid and another $50 in labor. Other misc. things will probably be in the $500-1000 worth of parts/labor if you pay someone to do it all and get it reliable and ready to face the next 100k miles. And, with all these projects, you could do one per year and still be ahead of the curve, considering you only seem to put 5k miles on it per year. I would probably do the timing belt now, but everything else could even wait another 20k miles, unless you mentioned suspension because it actually needs it NOW.
