Best budget preamp for my entry level system?

If you hadn't already purchased the amp, I would have suggested an integrated. Since you have the amp, and wish not to sell it, the PT-100 offers allot for the money; phonostage, DAC, headphone amp, and 3 additional line level inputs. 

For well under $300 you can get a B&K Reference 20 or even a Reference 30 preamp and, although not 2 channel, they perform exceptionally well in 2 channel. These originally sold for around $2500 and have a lot to offer. I own one of each along with a lot of other stuff and I am amazed at how they clear and detailed they are. 

To play vinyl you will need a phono pre. A Schiit Mani for $129 would serve perfectly.

the newer preamp has built in DAC, and related inputs. Nice, saves money, need for more cables both signal and power, and saves space:

 Unless you get the 'I need a better DAC bug', in which case, I would be happier with the matching style of the older Pre-amp.

Well, the USP-1 is ~10 years old. The PT-100 is new. Not knowing the comparative build quality or sonics hard to say which way to go. Likely the new one could be returned if it doesn't work out. Both seem to offer much at their price point.

Matching style is a consideration. Only the OP knows to what degree.  

The USP-1 is old, but it uses linear power supply and it looks like it even has a discrete audio stage.  The new PT-100 is all op-amps on a single board with switching power supply.  Also, Emotiva is not as cost effective as they used to be due to move a lot of their manufacturing away from China.  Ultimately, they just don't make them like they used to.

Older equipment is not always bad for 2-channel audio.  10 years is not a really long time.  I would start to get concerned in the 15-18 year area.  I have used and tested a lot of older equipment that beats the crap out of current year stuff.

Unfortunately, 2-channel preamps are always going to be more expensive than other technology and they tend to keep their value.  You could look at some older HT processors.  They were marketed at a very high price, but they lose their value extremely fast.  For example, the older Krell HTS 7.1 processor retailed for $8500, but was sold used for $2k about 12 years ago and can be gotten for around 500-800 today.  The KAV-280p pearmp of the same generation continued to sell used for around $2k even though it only retailed for about $4k, and still has a high $1200-1500 value today.  Point being is that you can get some pretty damn good preamp equipment by buying a old HT processor.  The B&K Ref 20 and Ref 50 mentioned above is an example.

If you look at audiogon, there are several around $300 or less:  Outlaw 990, Outlaw 970, Rotel RSP-1068.  The also have pretty good DACs.  The downside is they never have a phono input for turntables, so you would have to get an external phono preamp, but for the money it's an amazing deal.